13.2. 函数列表

QGIS中可用的函数、运算符和变量如下所示，并按类别分组。

13.2.1. 聚合函数

该组包含在图层和字段上聚合值的函数。

13.2.1.1. aggregate

返回使用其他图层中的要素计算的聚合值。

语法	aggregate(layer, aggregate, expression, [filter], [concatenator=''], [order_by]) [] 标记可选参数
参数	图层 - 字符串, 表示图层名称或图层ID 聚合 - 与要计算的聚合相对应的字符串。有效选项为: count count_distinct count_missing min max sum mean median stdev stdevsample range minority majority q1: 第一四分位数 q3: 第三四分位数 iqr: 四分位距 min_length: 最小字符串长度 max_length: 最大字符串长度 concatenate: 使用连接符连接字符串 concatenate_unique: 使用连接符连接唯一字符串 collect: 创建聚合的多部件几何图形 array_agg: create an array of aggregated values expression - sub expression or field name to aggregate filter - optional filter expression to limit the features used for calculating the aggregate. Fields and geometry are from the features on the joined layer. The source feature can be accessed with the variable @parent. concatenator - optional string to use to join values for 'concatenate' and 'concatenate_unique' aggregates order_by - optional filter expression to order the features used for calculating the aggregate. Fields and geometry are from the features on the joined layer. By default, the features will be returned in an unspecified order.
Examples	aggregate(layer:='rail_stations',aggregate:='sum',expression:="passengers") → sum of all values from the passengers field in the rail_stations layer aggregate('rail_stations','sum', "passengers"/7) → calculates a daily average of "passengers" by dividing the "passengers" field by 7 before summing the values aggregate(layer:='rail_stations',aggregate:='sum',expression:="passengers",filter:="class">3) → sums up all values from the "passengers" field from features where the "class" attribute is greater than 3 only aggregate(layer:='rail_stations',aggregate:='concatenate', expression:="name", concatenator:=',') → comma separated list of the name field for all features in the rail_stations layer aggregate(layer:='countries', aggregate:='max', expression:="code", filter:=intersects( $geometry, geometry(@parent) ) ) → The country code of an intersecting country on the layer 'countries' aggregate(layer:='rail_stations',aggregate:='sum',expression:="passengers",filter:=contains( @atlas_geometry, $geometry ) ) → sum of all values from the passengers field in the rail_stations within the current atlas feature aggregate(layer:='rail_stations', aggregate:='collect', expression:=centroid($geometry), filter:="region_name" = attribute(@parent,'name') ) → aggregates centroid geometries of the rail_stations of the same region as current feature

13.2.1.2. array_agg

Returns an array of aggregated values from a field or expression.

语法	array_agg(expression, [group_by], [filter], [order_by]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate order_by - optional expression to use to order features used to calculate aggregate. By default, the features will be returned in an unspecified order.
Examples	array_agg("name",group_by:="state") → list of name values, grouped by state field

13.2.1.3. collect

Returns the multipart geometry of aggregated geometries from an expression

语法	collect(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - geometry expression to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	collect( $geometry ) → multipart geometry of aggregated geometries collect( centroid($geometry), group_by:="region", filter:= "use" = 'civilian' ) → aggregated centroids of the civilian features based on their region value

13.2.1.4. concatenate

Returns all aggregated strings from a field or expression joined by a delimiter.

语法	concatenate(expression, [group_by], [filter], [concatenator], [order_by]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate concatenator - optional string to use to join values. Empty by default. order_by - optional expression to use to order features used to calculate aggregate. By default, the features will be returned in an unspecified order.
Examples	concatenate("town_name",group_by:="state",concatenator:=',') → comma separated list of town_names, grouped by state field

13.2.1.5. concatenate_unique

Returns all unique strings from a field or expression joined by a delimiter.

语法	concatenate_unique(expression, [group_by], [filter], [concatenator], [order_by]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate concatenator - optional string to use to join values. Empty by default. order_by - optional expression to use to order features used to calculate aggregate. By default, the features will be returned in an unspecified order.
Examples	concatenate_unique("town_name",group_by:="state",concatenator:=',') → comma separated list of unique town_names, grouped by state field

13.2.1.6. count

Returns the count of matching features.

语法	count(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	count("stations",group_by:="state") → count of stations, grouped by state field

13.2.1.7. count_distinct

Returns the count of distinct values.

语法	count_distinct(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	count_distinct("stations",group_by:="state") → count of distinct stations values, grouped by state field

13.2.1.8. count_missing

Returns the count of missing (NULL) values.

语法	count_missing(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	count_missing("stations",group_by:="state") → count of missing (NULL) station values, grouped by state field

13.2.1.9. iqr

Returns the calculated inter quartile range from a field or expression.

语法	iqr(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	iqr("population",group_by:="state") → inter quartile range of population value, grouped by state field

13.2.1.10. majority

Returns the aggregate majority of values (most commonly occurring value) from a field or expression.

语法	majority(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	majority("class",group_by:="state") → most commonly occurring class value, grouped by state field

13.2.1.11. max_length

Returns the maximum length of strings from a field or expression.

语法	max_length(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	max_length("town_name",group_by:="state") → maximum length of town_name, grouped by state field

13.2.1.12. maximum

Returns the aggregate maximum value from a field or expression.

语法	maximum(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	maximum("population",group_by:="state") → maximum population value, grouped by state field

13.2.1.13. mean

Returns the aggregate mean value from a field or expression.

语法	mean(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	mean("population",group_by:="state") → mean population value, grouped by state field

13.2.1.14. median

Returns the aggregate median value from a field or expression.

语法	median(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	median("population",group_by:="state") → median population value, grouped by state field

13.2.1.15. min_length

Returns the minimum length of strings from a field or expression.

语法	min_length(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	min_length("town_name",group_by:="state") → minimum length of town_name, grouped by state field

13.2.1.16. minimum

Returns the aggregate minimum value from a field or expression.

语法	minimum(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	minimum("population",group_by:="state") → minimum population value, grouped by state field

13.2.1.17. minority

Returns the aggregate minority of values (least occurring value) from a field or expression.

语法	minority(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	minority("class",group_by:="state") → least occurring class value, grouped by state field

13.2.1.18. q1

Returns the calculated first quartile from a field or expression.

语法	q1(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	q1("population",group_by:="state") → first quartile of population value, grouped by state field

13.2.1.19. q3

Returns the calculated third quartile from a field or expression.

语法	q3(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	q3("population",group_by:="state") → third quartile of population value, grouped by state field

13.2.1.20. range

Returns the aggregate range of values (maximum - minimum) from a field or expression.

语法	range(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	range("population",group_by:="state") → range of population values, grouped by state field

13.2.1.21. relation_aggregate

Returns an aggregate value calculated using all matching child features from a layer relation.

语法	relation_aggregate(relation, aggregate, expression, [concatenator=''], [order_by]) [] 标记可选参数
参数	relation - a string, representing a relation ID 聚合 - 与要计算的聚合相对应的字符串。有效选项为: count count_distinct count_missing min max sum mean median stdev stdevsample range minority majority q1: 第一四分位数 q3: 第三四分位数 iqr: 四分位距 min_length: 最小字符串长度 max_length: 最大字符串长度 concatenate: 使用连接符连接字符串 concatenate_unique: 使用连接符连接唯一字符串 collect: 创建聚合的多部件几何图形 array_agg: create an array of aggregated values expression - sub expression or field name to aggregate concatenator - optional string to use to join values for 'concatenate' aggregate order_by - optional expression to order the features used for calculating the aggregate. Fields and geometry are from the features on the joined layer. By default, the features will be returned in an unspecified order.
Examples	relation_aggregate(relation:='my_relation',aggregate:='mean',expression:="passengers") → mean value of all matching child features using the 'my_relation' relation relation_aggregate('my_relation','sum', "passengers"/7) → sum of the passengers field divided by 7 for all matching child features using the 'my_relation' relation relation_aggregate('my_relation','concatenate', "towns", concatenator:=',') → comma separated list of the towns field for all matching child features using the 'my_relation' relation relation_aggregate('my_relation','array_agg', "id") → array of the id field from all matching child features using the 'my_relation' relation

Further reading: Creating one or many to many relations

13.2.1.22. stdev

Returns the aggregate standard deviation value from a field or expression.

语法	stdev(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	stdev("population",group_by:="state") → standard deviation of population value, grouped by state field

13.2.1.23. sum

Returns the aggregate summed value from a field or expression.

语法	sum(expression, [group_by], [filter]) [] 标记可选参数
参数	expression - sub expression of field to aggregate group_by - optional expression to use to group aggregate calculations filter - optional expression to use to filter features used to calculate aggregate
Examples	sum("population",group_by:="state") → summed population value, grouped by state field

13.2.2. Array Functions

This group contains functions to create and manipulate arrays (also known as list data structures). The order of values within the array matters, unlike the 'map' data structure, where the order of key-value pairs is irrelevant and values are identified by their keys.

13.2.2.1. array

Returns an array containing all the values passed as parameter.

语法	array(value1, value2, ...)
参数	value - a value
Examples	array(2,10) → [ 2, 10 ] array(2,10)[0] → 2

13.2.2.2. array_all

Returns TRUE if an array contains all the values of a given array.

语法	array_all(array_a, array_b)
参数	array_a - an array array_b - the array of values to search
Examples	array_all(array(1,2,3),array(2,3)) → TRUE array_all(array(1,2,3),array(1,2,4)) → FALSE

13.2.2.3. array_append

Returns an array with the given value added at the end.

语法	array_append(array, value)
参数	array - an array value - the value to add
Examples	array_append(array(1,2,3),4) → [ 1, 2, 3, 4 ]

13.2.2.4. array_cat

Returns an array containing all the given arrays concatenated.

语法	array_cat(array1, array2, ...)
参数	array - an array
Examples	array_cat(array(1,2),array(2,3)) → [ 1, 2, 2, 3 ]

13.2.2.5. array_contains

Returns TRUE if an array contains the given value.

语法	array_contains(array, value)
参数	array - an array value - the value to search
Examples	array_contains(array(1,2,3),2) → TRUE

13.2.2.6. array_count

Counts the number of occurrences of a given value in an array.

语法	array_count(array, value)
参数	array - an array value - the value to count
Examples	array_count(array('a', 'b', 'c', 'b'), 'b') → 2

13.2.2.7. array_distinct

Returns an array containing distinct values of the given array.

语法	array_distinct(array)
参数	array - an array
Examples	array_distinct(array(1,2,3,2,1)) → [ 1, 2, 3 ]

13.2.2.8. array_filter

Returns an array with only the items for which the expression evaluates to true.

语法	array_filter(array, expression, [limit=0]) [] 标记可选参数
参数	array - an array expression - an expression to evaluate on each item. The variable @element will be replaced by the current value. limit - maximum number of elements to be returned. Use 0 to return all values.
Examples	array_filter(array(1,2,3),@element < 3) → [ 1, 2 ] array_filter(array(1,2,3),@element < 3, 1) → [ 1 ]

13.2.2.9. array_find

Returns the lowest index (0 for the first one) of a value within an array. Returns -1 if the value is not found.

语法	array_find(array, value)
参数	array - an array value - the value to search
Examples	array_find(array('a', 'b', 'c'), 'b') → 1 array_find(array('a', 'b', 'c', 'b'), 'b') → 1

13.2.2.10. array_first

Returns the first value of an array.

语法	array_first(array)
参数	array - an array
Examples	array_first(array('a','b','c')) → 'a'

13.2.2.11. array_foreach

Returns an array with the given expression evaluated on each item.

语法	array_foreach(array, expression)
参数	array - an array expression - an expression to evaluate on each item. The variable @element will be replaced by the current value.
Examples	array_foreach(array('a','b','c'),upper(@element)) → [ 'A', 'B', 'C' ] array_foreach(array(1,2,3),@element + 10) → [ 11, 12, 13 ]

13.2.2.12. array_get

Returns the Nth value (0 for the first one) or the last -Nth value (-1 for the last one) of an array.

语法	array_get(array, pos)
参数	array - an array pos - the index to get (0 based)
Examples	array_get(array('a','b','c'),1) → 'b' array_get(array('a','b','c'),-1) → 'c'

提示

You can also use the index operator ([]) to get a value from an array.

13.2.2.13. array_insert

Returns an array with the given value added at the given position.

语法	array_insert(array, pos, value)
参数	array - an array pos - the position where to add (0 based) value - the value to add
Examples	array_insert(array(1,2,3),1,100) → [ 1, 100, 2, 3 ]

13.2.2.14. array_intersect

Returns TRUE if at least one element of array1 exists in array2.

语法	array_intersect(array1, array2)
参数	array1 - an array array2 - another array
Examples	array_intersect(array(1,2,3,4),array(4,0,2,5)) → TRUE

13.2.2.15. array_last

Returns the last value of an array.

语法	array_last(array)
参数	array - an array
Examples	array_last(array('a','b','c')) → 'c'

13.2.2.16. array_length

Returns the number of elements of an array.

语法	array_length(array)
参数	array - an array
Examples	array_length(array(1,2,3)) → 3

13.2.2.17. array_majority

Returns the most common values in an array.

语法	array_majority(array, [option='all']) [] 标记可选参数
参数	array - an array option='all' - a string specifying the return values handling. Valid options are: all: Default, all most common values are returned in an array. any: Returns one of the most common values. median: Returns the median of the most common values. Non arithmetic values are ignored. real_majority: Returns the value which occurs more than half the size of the array.
Examples	array_majority(array(0,1,42,42,43), 'all') → [ 42 ] array_majority(array(0,1,42,42,43,1), 'all') → [ 42, 1 ] array_majority(array(0,1,42,42,43,1), 'any') → 1 or 42 array_majority(array(0,1,1,2,2), 'median') → 1.5 array_majority(array(0,1,42,42,43), 'real_majority') → NULL array_majority(array(0,1,42,42,43,42), 'real_majority') → NULL array_majority(array(0,1,42,42,43,42,42), 'real_majority') → 42

13.2.2.18. array_max

Returns the maximum value of an array.

语法	array_max(array)
参数	array - an array
Examples	array_max(array(0,42,4,2)) → 42

13.2.2.19. array_mean

Returns the mean of arithmetic values in an array. Non numeric values in the array are ignored.

语法	array_mean(array)
参数	array - an array
Examples	array_mean(array(0,1,7,66.6,135.4)) → 42 array_mean(array(0,84,'a','b','c')) → 42

13.2.2.20. array_median

Returns the median of arithmetic values in an array. Non arithmetic values in the array are ignored.

语法	array_median(array)
参数	array - an array
Examples	array_median(array(0,1,42,42,43)) → 42 array_median(array(0,1,2,42,'a','b')) → 1.5

13.2.2.21. array_min

Returns the minimum value of an array.

语法	array_min(array)
参数	array - an array
Examples	array_min(array(43,42,54)) → 42

13.2.2.22. array_minority

Returns the less common values in an array.

语法	array_minority(array, [option='all']) [] 标记可选参数
参数	array - an array option='all' - a string specifying the return values handling. Valid options are: all: Default, all less common values are returned in an array. any: Returns one of the less common values. median: Returns the median of the less common values. Non arithmetic values are ignored. real_minority: Returns values which occur less than half the size of the array.
Examples	array_minority(array(0,42,42), 'all') → [ 0 ] array_minority(array(0,1,42,42), 'all') → [ 0, 1 ] array_minority(array(0,1,42,42,43,1), 'any') → 0 or 43 array_minority(array(1,2,3,3), 'median') → 1.5 array_minority(array(0,1,42,42,43), 'real_minority') → [ 42, 43, 0, 1 ] array_minority(array(0,1,42,42,43,42), 'real_minority') → [ 42, 43, 0, 1 ] array_minority(array(0,1,42,42,43,42,42), 'real_minority') → [ 43, 0, 1 ]

13.2.2.23. array_prepend

Returns an array with the given value added at the beginning.

语法	array_prepend(array, value)
参数	array - an array value - the value to add
Examples	array_prepend(array(1,2,3),0) → [ 0, 1, 2, 3 ]

13.2.2.24. array_prioritize

Returns an array sorted using the ordering specified in another array. Values which are present in the first array but are missing from the second array will be added to the end of the result.

语法	array_prioritize(array, array_prioritize)
参数	array - an array array_prioritize - an array with values ordered by priority
Examples	array_prioritize(array(1, 8, 2, 5), array(5, 4, 2, 1, 3, 8)) → [ 5, 2, 1, 8 ] array_prioritize(array(5, 4, 2, 1, 3, 8), array(1, 8, 6, 5)) → [ 1, 8, 5, 4, 2, 3 ]

13.2.2.25. array_remove_all

Returns an array with all the entries of the given value removed.

语法	array_remove_all(array, value)
参数	array - an array value - the values to remove
Examples	array_remove_all(array('a','b','c','b'),'b') → [ 'a', 'c' ]

13.2.2.26. array_remove_at

Returns an array with the item at the given index removed. Supports positive (0 for the first element) and negative (the last -Nth value, -1 for the last element) index.

语法	array_remove_at(array, pos)
参数	array - an array pos - the position to remove (0 based)
Examples	array_remove_at(array(1, 2, 3), 1) → [1, 3 ] array_remove_at(array(1, 2, 3), -1) → [1, 2 ]

13.2.2.27. array_replace

Returns an array with the supplied value, array, or map of values replaced.

Value & array variant

Returns an array with the supplied value or array of values replaced by another value or an array of values.

语法	array_replace(array, before, after)
参数	array - the input array before - the value or array of values to replace after - the value or array of values to use as a replacement
Examples	array_replace(array('QGIS','SHOULD','ROCK'),'SHOULD','DOES') → [ 'QGIS', 'DOES', 'ROCK' ] array_replace(array(3,2,1),array(1,2,3),array(7,8,9)) → [ 9, 8, 7 ] array_replace(array('Q','G','I','S'),array('Q','S'),'-') → [ '-', 'G', 'I', '-' ]

Map variant

Returns an array with the supplied map keys replaced by their paired values.

语法	array_replace(array, map)
参数	array - the input array map - the map containing keys and values
Examples	array_replace(array('APP', 'SHOULD', 'ROCK'),map('APP','QGIS','SHOULD','DOES')) → [ 'QGIS', 'DOES', 'ROCK' ]

13.2.2.28. array_reverse

Returns the given array with array values in reversed order.

语法	array_reverse(array)
参数	array - an array
Examples	array_reverse(array(2,4,0,10)) → [ 10, 0, 4, 2 ]

13.2.2.29. array_slice

Returns a portion of the array. The slice is defined by the start_pos and end_pos arguments.

语法	array_slice(array, start_pos, end_pos)
参数	array - an array start_pos - the index of the start position of the slice (0 based). The start_pos index is included in the slice. If you use a negative start_pos, the index is counted from the end of the list (-1 based). end_pos - the index of the end position of the slice (0 based). The end_pos index is included in the slice. If you use a negative end_pos, the index is counted from the end of the list (-1 based).
Examples	array_slice(array(1,2,3,4,5),0,3) → [ 1, 2, 3, 4 ] array_slice(array(1,2,3,4,5),0,-1) → [ 1, 2, 3, 4, 5 ] array_slice(array(1,2,3,4,5),-5,-1) → [ 1, 2, 3, 4, 5 ] array_slice(array(1,2,3,4,5),0,0) → [ 1 ] array_slice(array(1,2,3,4,5),-2,-1) → [ 4, 5 ] array_slice(array(1,2,3,4,5),-1,-1) → [ 5 ] array_slice(array('Dufour','Valmiera','Chugiak','Brighton'),1,2) → [ 'Valmiera', 'Chugiak' ] array_slice(array('Dufour','Valmiera','Chugiak','Brighton'),-2,-1) → [ 'Chugiak', 'Brighton' ]

13.2.2.30. array_sort

Returns the provided array with its elements sorted.

语法	array_sort(array, [ascending=true]) [] 标记可选参数
参数	array - an array ascending - set this parameter to false to sort the array in descending order
Examples	array_sort(array(3,2,1)) → [ 1, 2, 3 ]

13.2.2.31. array_sum

Returns the sum of arithmetic values in an array. Non numeric values in the array are ignored.

语法	array_sum(array)
参数	array - an array
Examples	array_sum(array(0,1,39.4,1.6,'a')) → 42.0

13.2.2.32. array_to_string

Concatenates array elements into a string separated by a delimiter and using optional string for empty values.

语法	array_to_string(array, [delimiter=','], [empty_value='']) [] 标记可选参数
参数	array - the input array delimiter - the string delimiter used to separate concatenated array elements empty_value - the optional string to use as replacement for empty (zero length) matches
Examples	array_to_string(array('1','2','3')) → '1,2,3' array_to_string(array(1,2,3),'-') → '1-2-3' array_to_string(array('1','','3'),',','0') → '1,0,3'

13.2.2.33. generate_series

Creates an array containing a sequence of numbers.

语法	generate_series(start, stop, [step=1]) [] 标记可选参数
参数	start - first value of the sequence stop - value that ends the sequence once reached step - value used as the increment between values
Examples	generate_series(1,5) → [ 1, 2, 3, 4, 5 ] generate_series(5,1,-1) → [ 5, 4, 3, 2, 1 ]

13.2.2.34. geometries_to_array

Splits a geometry into simpler geometries in an array.

语法	geometries_to_array(geometry)
参数	geometry - the input geometry
Examples	geometries_to_array(geom_from_wkt('GeometryCollection (Polygon ((5 8, 4 1, 3 2, 5 8)),LineString (3 2, 4 2))')) → an array of a polygon and a line geometries geom_to_wkt(geometries_to_array(geom_from_wkt('GeometryCollection (Polygon ((5 8, 4 1, 3 2, 5 8)),LineString (3 2, 4 2))'))[0]) → 'Polygon ((5 8, 4 1, 3 2, 5 8))' geometries_to_array(geom_from_wkt('MULTIPOLYGON(((5 5,0 0,0 10,5 5)),((5 5,10 10,10 0,5 5))')) → an array of two polygon geometries

13.2.2.35. regexp_matches

Returns an array of all strings captured by capturing groups, in the order the groups themselves appear in the supplied regular expression against a string.

语法	regexp_matches(string, regex, [empty_value='']) [] 标记可选参数
参数	string - the string to capture groups from against the regular expression regex - the regular expression used to capture groups empty_value - the optional string to use as replacement for empty (zero length) matches
Examples	regexp_matches('QGIS=>rocks','(.*)=>(.*)') → [ 'QGIS', 'rocks' ] regexp_matches('key=>','(.*)=>(.*)','empty value') → [ 'key', 'empty value' ]

13.2.2.36. string_to_array

Splits string into an array using supplied delimiter and optional string for empty values.

语法	string_to_array(string, [delimiter=','], [empty_value='']) [] 标记可选参数
参数	string - the input string delimiter - the string delimiter used to split the input string empty_value - the optional string to use as replacement for empty (zero length) matches
Examples	string_to_array('1,2,3',',') → [ '1', '2', '3' ] string_to_array('1,,3',',','0') → [ '1', '0', '3' ]

13.2.3. Color Functions

This group contains functions for manipulating colors.

13.2.3.1. color_cmyk

Returns a string representation of a color based on its cyan, magenta, yellow and black components

语法	color_cmyk(cyan, magenta, yellow, black)
参数	cyan - cyan component of the color, as a percentage integer value from 0 to 100 magenta - magenta component of the color, as a percentage integer value from 0 to 100 yellow - yellow component of the color, as a percentage integer value from 0 to 100 black - black component of the color, as a percentage integer value from 0 to 100
Examples	color_cmyk(100,50,0,10) → '0,115,230'

13.2.3.2. color_cmyka

Returns a string representation of a color based on its cyan, magenta, yellow, black and alpha (transparency) components

语法	color_cmyka(cyan, magenta, yellow, black, alpha)
参数	cyan - cyan component of the color, as a percentage integer value from 0 to 100 magenta - magenta component of the color, as a percentage integer value from 0 to 100 yellow - yellow component of the color, as a percentage integer value from 0 to 100 black - black component of the color, as a percentage integer value from 0 to 100 alpha - alpha component as an integer value from 0 (completely transparent) to 255 (opaque).
Examples	color_cmyka(100,50,0,10,200) → '0,115,230,200'

13.2.3.3. color_grayscale_average

Applies a grayscale filter and returns a string representation from a provided color.

语法	color_grayscale_average(color)
参数	color - a color string
Examples	color_grayscale_average('255,100,50') → '135,135,135,255'

13.2.3.4. color_hsl

Returns a string representation of a color based on its hue, saturation, and lightness attributes.

语法	color_hsl(hue, saturation, lightness)
参数	hue - hue of the color, as an integer value from 0 to 360 saturation - saturation percentage of the color as an integer value from 0 to 100 lightness - lightness percentage of the color as an integer value from 0 to 100
Examples	color_hsl(100,50,70) → '166,217,140'

13.2.3.5. color_hsla

Returns a string representation of a color based on its hue, saturation, lightness and alpha (transparency) attributes

语法	color_hsla(hue, saturation, lightness, alpha)
参数	hue - hue of the color, as an integer value from 0 to 360 saturation - saturation percentage of the color as an integer value from 0 to 100 lightness - lightness percentage of the color as an integer value from 0 to 100 alpha - alpha component as an integer value from 0 (completely transparent) to 255 (opaque).
Examples	color_hsla(100,50,70,200) → '166,217,140,200'

13.2.3.6. color_hsv

Returns a string representation of a color based on its hue, saturation, and value attributes.

语法	color_hsv(hue, saturation, value)
参数	hue - hue of the color, as an integer value from 0 to 360 saturation - saturation percentage of the color as an integer value from 0 to 100 value - value percentage of the color as an integer from 0 to 100
Examples	color_hsv(40,100,100) → '255,170,0'

13.2.3.7. color_hsva

Returns a string representation of a color based on its hue, saturation, value and alpha (transparency) attributes.

语法	color_hsva(hue, saturation, value, alpha)
参数	hue - hue of the color, as an integer value from 0 to 360 saturation - saturation percentage of the color as an integer value from 0 to 100 value - value percentage of the color as an integer from 0 to 100 alpha - alpha component as an integer value from 0 (completely transparent) to 255 (opaque)
Examples	color_hsva(40,100,100,200) → '255,170,0,200'

13.2.3.8. color_mix_rgb

Returns a string representing a color mixing the red, green, blue, and alpha values of two provided colors based on a given ratio.

语法	color_mix_rgb(color1, color2, ratio)
参数	color1 - a color string color2 - a color string ratio - a ratio
Examples	color_mix_rgb('0,0,0','255,255,255',0.5) → '127,127,127,255'

13.2.3.9. color_part

Returns a specific component from a color string, e.g., the red component or alpha component.

语法	color_part(color, component)
参数	color - a color string component - a string corresponding to the color component to return. Valid options are: red: RGB red component (0-255) green: RGB green component (0-255) blue: RGB blue component (0-255) alpha: alpha (transparency) value (0-255) hue: HSV hue (0-360) saturation: HSV saturation (0-100) value: HSV value (0-100) hsl_hue: HSL hue (0-360) hsl_saturation: HSL saturation (0-100) lightness: HSL lightness (0-100) cyan: CMYK cyan component (0-100) magenta: CMYK magenta component (0-100) yellow: CMYK yellow component (0-100) black: CMYK black component (0-100)
Examples	color_part('200,10,30','green') → 10

13.2.3.10. color_rgb

Returns a string representation of a color based on its red, green, and blue components.

语法	color_rgb(red, green, blue)
参数	red - red component as an integer value from 0 to 255 green - green component as an integer value from 0 to 255 blue - blue component as an integer value from 0 to 255
Examples	color_rgb(255,127,0) → '255,127,0'

13.2.3.11. color_rgba

Returns a string representation of a color based on its red, green, blue, and alpha (transparency) components.

语法	color_rgba(red, green, blue, alpha)
参数	red - red component as an integer value from 0 to 255 green - green component as an integer value from 0 to 255 blue - blue component as an integer value from 0 to 255 alpha - alpha component as an integer value from 0 (completely transparent) to 255 (opaque).
Examples	color_rgba(255,127,0,200) → '255,127,0,200'

13.2.3.12. create_ramp

Returns a gradient ramp from a map of color strings and steps.

语法	create_ramp(map, [discrete=false]) [] 标记可选参数
参数	map - a map of color strings and steps discrete - set this parameter to true to create a discrete color ramp
Examples	ramp_color(create_ramp(map(0,'0,0,0',1,'255,0,0')),1) → '255,0,0,255'

13.2.3.13. darker

Returns a darker (or lighter) color string

语法	darker(color, factor)
参数	color - a color string factor - an integer corresponding to the darkening factor: if the factor is greater than 100, this function returns a darker color (e.g., setting factor to 200 returns a color that is half the brightness); if the factor is less than 100, the return color is lighter, but using the lighter() function for this purpose is recommended; if the factor is 0 or negative, the return value is unspecified.
Examples	darker('200,10,30', 200) → '100,5,15,255'

Further reading: lighter

13.2.3.14. lighter

Returns a lighter (or darker) color string

语法	lighter(color, factor)
参数	color - a color string factor - an integer corresponding to the lightening factor: if the factor is greater than 100, this function returns a lighter color (e.g., setting factor to 150 returns a color that is 50% brighter); if the factor is less than 100, the return color is darker, but using the darker() function for this purpose is recommended; if the factor is 0 or negative, the return value is unspecified.
Examples	lighter('200,10,30', 200) → '255,158,168,255'

Further reading: darker

13.2.3.15. project_color

Returns a color from the project's color scheme.

语法	project_color(name)
参数	name - a color name
Examples	project_color('Logo color') → '20,140,50'

Further reading: setting project colors

13.2.3.16. ramp_color

Returns a string representing a color from a color ramp.

Saved ramp variant

Returns a string representing a color from a saved ramp

语法	ramp_color(ramp_name, value)
参数	ramp_name - the name of the color ramp as a string, for example 'Spectral' value - the position on the ramp to select the color from as a real number between 0 and 1
Examples	ramp_color('Spectral',0.3) → '253,190,115,255'

备注

The color ramps available vary between QGIS installations. This function may not give the expected results if you move your QGIS project between installations.

Expression-created ramp variant

Returns a string representing a color from an expression-created ramp

语法	ramp_color(ramp, value)
参数	ramp - the color ramp value - the position on the ramp to select the color from as a real number between 0 and 1
Examples	ramp_color(create_ramp(map(0,'0,0,0',1,'255,0,0')),1) → '255,0,0,255'

Further reading: 设置颜色渐变, 颜色渐变下拉快捷方式

13.2.3.17. set_color_part

Sets a specific color component for a color string, e.g., the red component or alpha component.

语法	set_color_part(color, component, value)
参数	color - a color string component - a string corresponding to the color component to set. Valid options are: red: RGB red component (0-255) green: RGB green component (0-255) blue: RGB blue component (0-255) alpha: alpha (transparency) value (0-255) hue: HSV hue (0-360) saturation: HSV saturation (0-100) value: HSV value (0-100) hsl_hue: HSL hue (0-360) hsl_saturation: HSL saturation (0-100) lightness: HSL lightness (0-100) cyan: CMYK cyan component (0-100) magenta: CMYK magenta component (0-100) yellow: CMYK yellow component (0-100) black: CMYK black component (0-100) value - new value for color component, respecting the ranges listed above
Examples	set_color_part('200,10,30','green',50) → '200,50,30,255'

13.2.4. Conditional Functions

This group contains functions to handle conditional checks in expressions.

13.2.4.1. CASE

CASE is used to evaluate a series of conditions and return a result for the first condition met. The conditions are evaluated sequentially, and if a condition is true, the evaluation stops, and the corresponding result is returned. If none of the conditions are true, the value in the ELSE clause is returned. Furthermore, if no ELSE clause is set and none of the conditions are met, NULL is returned.

CASE

WHEN condition THEN result

[ …n ]

[ ELSE result ]

END

[ ] marks optional components

参数	WHEN condition - A condition expression to evaluate THEN result - If condition evaluates to True then result is evaluated and returned. ELSE result - If none of the above conditions evaluated to True then result is evaluated and returned.
Examples	CASE WHEN "name" IS NULL THEN 'None' END → Returns the string 'None' if the "name" field is NULL CASE WHEN $area > 10000 THEN 'Big property' WHEN $area > 5000 THEN 'Medium property' ELSE 'Small property' END → Returns the string 'Big property' if the area is bigger than 10000, 'Medium property' if the area is between 5000 and 10000, and 'Small property' for others

13.2.4.2. coalesce

Returns the first non-NULL value from the expression list.

This function can take any number of arguments.

语法	coalesce(expression1, expression2, ...)
参数	expression - any valid expression or value, regardless of type.
Examples	coalesce(NULL, 2) → 2 coalesce(NULL, 2, 3) → 2 coalesce(7, NULL, 3*2) → 7 coalesce("fieldA", "fallbackField", 'ERROR') → value of fieldA if it is non-NULL else the value of "fallbackField" or the string 'ERROR' if both are NULL

13.2.4.3. if

Tests a condition and returns a different result depending on the conditional check.

语法	if(condition, result_when_true, result_when_false)
参数	condition - the condition which should be checked result_when_true - the result which will be returned when the condition is true or another value that does not convert to false. result_when_false - the result which will be returned when the condition is false or another value that converts to false like 0 or ''. NULL will also be converted to false.
Examples	if( 1+1=2, 'Yes', 'No' ) → 'Yes' if( 1+1=3, 'Yes', 'No' ) → 'No' if( 5 > 3, 1, 0) → 1 if( '', 'It is true (not empty)', 'It is false (empty)' ) → 'It is false (empty)' if( ' ', 'It is true (not empty)', 'It is false (empty)' ) → 'It is true (not empty)' if( 0, 'One', 'Zero' ) → 'Zero' if( 10, 'One', 'Zero' ) → 'One'

13.2.4.4. nullif

Returns a NULL value if value1 equals value2; otherwise it returns value1. This can be used to conditionally substitute values with NULL.

语法	nullif(value1, value2)
参数	value1 - The value that should either be used or substituted with NULL. value2 - The control value that will trigger the NULL substitution.
Examples	nullif('(none)', '(none)') → NULL nullif('text', '(none)') → 'text' nullif("name", '') → NULL, if name is an empty string (or already NULL), the name in any other case.

13.2.4.5. regexp_match

Return the first matching position matching a regular expression within an unicode string, or 0 if the substring is not found.

语法	regexp_match(input_string, regex)
参数	input_string - the string to test against the regular expression regex - The regular expression to test against. Backslash characters must be double escaped (e.g., "\\s" to match a white space character or "\\b" to match a word boundary).
Examples	regexp_match('QGIS ROCKS','\\sROCKS') → 5 regexp_match('Budač','udač\\b') → 2

13.2.4.6. try

Tries an expression and returns its value if error-free. If the expression returns an error, an alternative value will be returned when provided otherwise the function will return NULL.

语法	try(expression, [alternative]) [] 标记可选参数
参数	expression - the expression which should be run alternative - the result which will be returned if the expression returns an error.
Examples	try( to_int( '1' ), 0 ) → 1 try( to_int( 'a' ), 0 ) → 0 try( to_date( 'invalid_date' ) ) → NULL

13.2.5. Conversions Functions

This group contains functions to convert one data type to another (e.g., string from/to integer, binary from/to string, string to date, ...).

13.2.5.1. from_base64

Decodes a string in the Base64 encoding into a binary value.

语法	from_base64(string)
参数	string - the string to decode
Examples	from_base64('UUdJUw==') → 'QGIS'

13.2.5.2. hash

Creates a hash from a string with a given method. One byte (8 bits) is represented with two hex ''digits'', so 'md4' (16 bytes) produces a 16 * 2 = 32 character long hex string and 'keccak_512' (64 bytes) produces a 64 * 2 = 128 character long hex string.

语法	hash(string, method)
参数	string - the string to hash method - The hash method among 'md4', 'md5', 'sha1', 'sha224', 'sha384', 'sha512', 'sha3_224', 'sha3_256', 'sha3_384', 'sha3_512', 'keccak_224', 'keccak_256', 'keccak_384', 'keccak_512'
Examples	hash('QGIS', 'md4') → 'c0fc71c241cdebb6e888cbac0e2b68eb' hash('QGIS', 'md5') → '57470aaa9e22adaefac7f5f342f1c6da' hash('QGIS', 'sha1') → 'f87cfb2b74cdd5867db913237024e7001e62b114' hash('QGIS', 'sha224') → '4093a619ada631c770f44bc643ead18fb393b93d6a6af1861fcfece0' hash('QGIS', 'sha256') → 'eb045cba7a797aaa06ac58830846e40c8e8c780bc0676d3393605fae50c05309' hash('QGIS', 'sha384') → '91c1de038cc3d09fdd512e99f9dd9922efadc39ed21d3922e69a4305cc25506033aee388e554b78714c8734f9cd7e610' hash('QGIS', 'sha512') → 'c2c092f2ab743bf8edbeb6d028a745f30fc720408465ed369421f0a4e20fa5e27f0c90ad72d3f1d836eaa5d25cd39897d4cf77e19984668ef58da6e3159f18ac' hash('QGIS', 'sha3_224') → '467f49a5039e7280d5d42fd433e80d203439e338eaabd701f0d6c17d' hash('QGIS', 'sha3_256') → '540f7354b6b8a6e735f2845250f15f4f3ba4f666c55574d9e9354575de0e980f' hash('QGIS', 'sha3_384') → '96052da1e77679e9a65f60d7ead961b287977823144786386eb43647b0901fd8516fa6f1b9d243fb3f28775e6dde6107' hash('QGIS', 'sha3_512') → '900d079dc69761da113980253aa8ac0414a8bd6d09879a916228f8743707c4758051c98445d6b8945ec854ff90655005e02aceb0a2ffc6a0ebf818745d665349' hash('QGIS', 'keccak_224') → '5b0ce6acef8b0a121d4ac4f3eaa8503c799ad4e26a3392d1fb201478' hash('QGIS', 'keccak_256') → '991c520aa6815392de24087f61b2ae0fd56abbfeee4a8ca019c1011d327c577e' hash('QGIS', 'keccak_384') → 'c57a3aed9d856fa04e5eeee9b62b6e027cca81ba574116d3cc1f0d48a1ef9e5886ff463ea8d0fac772ee473bf92f810d'

13.2.5.3. md5

Creates a md5 hash from a string.

语法	md5(string)
参数	string - the string to hash
Examples	md5('QGIS') → '57470aaa9e22adaefac7f5f342f1c6da'

13.2.5.4. sha256

Creates a sha256 hash from a string.

语法	sha256(string)
参数	string - the string to hash
Examples	sha256('QGIS') → 'eb045cba7a797aaa06ac58830846e40c8e8c780bc0676d3393605fae50c05309'

13.2.5.5. to_base64

Encodes a binary value into a string, using the Base64 encoding.

语法	to_base64(value)
参数	value - the binary value to encode
Examples	to_base64('QGIS') → 'UUdJUw=='

13.2.5.6. to_date

Converts a string into a date object. An optional format string can be provided to parse the string; see QDate::fromString or the documentation of the format_date function for additional documentation on the format. By default the current QGIS user locale is used.

语法	to_date(string, [format], [language]) [] 标记可选参数
参数	string - string representing a date value format - format used to convert the string into a date language - language (lowercase, two- or three-letter, ISO 639 language code) used to convert the string into a date. By default the current QGIS user locale is used.
Examples	to_date('2012-05-04') → 2012-05-04 to_date('June 29, 2019','MMMM d, yyyy') → 2019-06-29, if the current locale uses the name 'June' for the sixth month, otherwise an error occurs to_date('29 juin, 2019','d MMMM, yyyy','fr') → 2019-06-29

13.2.5.7. to_datetime

Converts a string into a datetime object. An optional format string can be provided to parse the string; see QDate::fromString, QTime::fromString or the documentation of the format_date function for additional documentation on the format. By default the current QGIS user locale is used.

语法	to_datetime(string, [format], [language]) [] 标记可选参数
参数	string - string representing a datetime value format - format used to convert the string into a datetime language - language (lowercase, two- or three-letter, ISO 639 language code) used to convert the string into a datetime. By default the current QGIS user locale is used.
Examples	to_datetime('2012-05-04 12:50:00') → 2012-05-04T12:50:00 to_datetime('June 29, 2019 @ 12:34','MMMM d, yyyy @ HH:mm') → 2019-06-29T12:34, if the current locale uses the name 'June' for the sixth month, otherwise an error occurs to_datetime('29 juin, 2019 @ 12:34','d MMMM, yyyy @ HH:mm','fr') → 2019-06-29T12:34

13.2.5.8. to_decimal

Converts a degree, minute, second coordinate to its decimal equivalent.

语法	to_decimal(value)
参数	value - A degree, minute, second string.
Examples	to_decimal('6°21\'16.445') → 6.3545680555

13.2.5.9. to_dm

Converts a coordinate to degree, minute.

语法	to_dm(coordinate, axis, precision, [formatting=]) [] 标记可选参数
参数	coordinate - A latitude or longitude value. axis - The axis of the coordinate. Either 'x' or 'y'. precision - Number of decimals. formatting - Designates the formatting type. Acceptable values are NULL (default), 'aligned' or 'suffix'.
Examples	to_dm(6.1545681, 'x', 3) → 6°9.274′ to_dm(6.1545681, 'y', 4, 'aligned') → 6°09.2741′N to_dm(6.1545681, 'y', 4, 'suffix') → 6°9.2741′N

13.2.5.10. to_dms

Converts a coordinate to degree, minute, second.

语法	to_dms(coordinate, axis, precision, [formatting=]) [] 标记可选参数
参数	coordinate - A latitude or longitude value. axis - The axis of the coordinate. Either 'x' or 'y'. precision - Number of decimals. formatting - Designates the formatting type. Acceptable values are NULL (default), 'aligned' or 'suffix'.
Examples	to_dms(6.1545681, 'x', 3) → 6°9′16.445″ to_dms(6.1545681, 'y', 4, 'aligned') → 6°09′16.4452″N to_dms(6.1545681, 'y', 4, 'suffix') → 6°9′16.4452″N

13.2.5.11. to_int

Converts a string to integer number. Nothing is returned if a value cannot be converted to integer (e.g '123asd' is invalid).

语法	to_int(string)
参数	string - string to convert to integer number
Examples	to_int('123') → 123

13.2.5.12. to_interval

Converts a string to an interval type. Can be used to take days, hours, month, etc of a date.

语法	to_interval(string)
参数	string - a string representing an interval. Allowable formats include {n} days {n} hours {n} months.
Examples	to_interval('1 day 2 hours') → interval: 1.08333 days to_interval( '0.5 hours' ) → interval: 30 minutes to_datetime('2012-05-05 12:00:00') - to_interval('1 day 2 hours') → 2012-05-04T10:00:00

13.2.5.13. to_real

Converts a string to a real number. Nothing is returned if a value cannot be converted to real (e.g '123.56asd' is invalid). Numbers are rounded after saving changes if the precision is smaller than the result of the conversion.

语法	to_real(string)
参数	string - string to convert to real number
Examples	to_real('123.45') → 123.45

13.2.5.14. to_string

Converts a number to string.

语法	to_string(number)
参数	number - Integer or real value. The number to convert to string.
Examples	to_string(123) → '123'

13.2.5.15. to_time

Converts a string into a time object. An optional format string can be provided to parse the string; see QTime::fromString for additional documentation on the format.

语法	to_time(string, [format], [language]) [] 标记可选参数
参数	string - string representing a time value format - format used to convert the string into a time language - language (lowercase, two- or three-letter, ISO 639 language code) used to convert the string into a time
Examples	to_time('12:30:01') → 12:30:01 to_time('12:34','HH:mm') → 12:34:00 to_time('12:34','HH:mm','fr') → 12:34:00

13.2.6. Custom Functions

This group contains functions created by the user. See Function Editor for more details.

13.2.7. Date and Time Functions

This group contains functions for handling date and time data. This group shares several functions with the Conversions Functions (to_date, to_time, to_datetime, to_interval) and String Functions (format_date) groups.

备注

Storing date, datetime and intervals on fields

The ability to store date, time and datetime values directly on fields depends on the data source's provider (e.g., Shapefile accepts date format, but not datetime or time format). The following are some suggestions to overcome this limitation:

• date, datetime and time can be converted and stored in text type fields using the format_date() function.

• Intervals can be stored in integer or decimal type fields after using one of the date extraction functions (e.g., day() to get the interval expressed in days)

13.2.7.1. age

Returns the difference between two dates or datetimes.

The difference is returned as an Interval and needs to be used with one of the following functions in order to extract useful information:

• year

• month

• week

• day

• hour

• minute

• second

语法	age(datetime1, datetime2)
参数	datetime1 - a string, date or datetime representing the later date datetime2 - a string, date or datetime representing the earlier date
Examples	day(age('2012-05-12','2012-05-02')) → 10 hour(age('2012-05-12','2012-05-02')) → 240

13.2.7.2. datetime_from_epoch

Returns a datetime whose date and time are the number of milliseconds, msecs, that have passed since 1970-01-01T00:00:00.000, Coordinated Universal Time (Qt.UTC), and converted to Qt.LocalTime.

语法	datetime_from_epoch(int)
参数	int - number (milliseconds)
Examples	datetime_from_epoch(1483225200000) → 2017-01-01T00:00:00

13.2.7.3. day

Extracts the day from a date, or the number of days from an interval.

Date variant

Extracts the day from a date or datetime.

语法	day(date)
参数	date - a date or datetime value
Examples	day('2012-05-12') → 12

Interval variant

Calculates the length in days of an interval.

语法	day(interval)
参数	interval - interval value to return number of days from
Examples	day(to_interval('3 days')) → 3 day(to_interval('3 weeks 2 days')) → 23 day(age('2012-01-01','2010-01-01')) → 730

13.2.7.4. day_of_week

Returns the day of the week for a specified date or datetime. The returned value ranges from 0 to 6, where 0 corresponds to a Sunday and 6 to a Saturday.

语法	day_of_week(date)
参数	date - date or datetime value
Examples	day_of_week(to_date('2015-09-21')) → 1

13.2.7.5. epoch

Returns the interval in milliseconds between the unix epoch and a given date value.

语法	epoch(date)
参数	date - a date or datetime value
Examples	epoch(to_date('2017-01-01')) → 1483203600000

13.2.7.6. format_date

Formats a date type or string into a custom string format. Uses Qt date/time format strings. See QDateTime::toString.

语法	format_date(datetime, format, [language]) [] 标记可选参数
参数	datetime - date, time or datetime value format - String template used to format the string. Expression Output d the day as number without a leading zero (1 to 31) dd the day as number with a leading zero (01 to 31) ddd the abbreviated localized day name (e.g. 'Mon' to 'Sun') dddd the long localized day name (e.g. 'Monday' to 'Sunday') M the month as number without a leading zero (1-12) MM the month as number with a leading zero (01-12) MMM the abbreviated localized month name (e.g. 'Jan' to 'Dec') MMMM the long localized month name (e.g. 'January' to 'December') yy the year as two digit number (00-99) yyyy the year as four digit number These expressions may be used for the time part of the format string: Expression Output h the hour without a leading zero (0 to 23 or 1 to 12 if AM/PM display) hh the hour with a leading zero (00 to 23 or 01 to 12 if AM/PM display) H the hour without a leading zero (0 to 23, even with AM/PM display) HH the hour with a leading zero (00 to 23, even with AM/PM display) m the minute without a leading zero (0 to 59) mm the minute with a leading zero (00 to 59) s the second without a leading zero (0 to 59) ss the second with a leading zero (00 to 59) z the milliseconds without trailing zeroes (0 to 999) zzz the milliseconds with trailing zeroes (000 to 999) AP or A interpret as an AM/PM time. AP must be either 'AM' or 'PM'. ap or a Interpret as an AM/PM time. ap must be either 'am' or 'pm'. language - language (lowercase, two- or three-letter, ISO 639 language code) used to format the date into a custom string. By default the current QGIS user locale is used.
Examples	format_date('2012-05-15','dd.MM.yyyy') → '15.05.2012' format_date('2012-05-15','d MMMM yyyy','fr') → '15 mai 2012' format_date('2012-05-15','dddd') → 'Tuesday', if the current locale is an English variant format_date('2012-05-15 13:54:20','dd.MM.yy') → '15.05.12' format_date('13:54:20','hh:mm AP') → '01:54 PM'

13.2.7.7. hour

Extracts the hour part from a datetime or time, or the number of hours from an interval.

Time variant

Extracts the hour part from a time or datetime.

语法	hour(datetime)
参数	datetime - a time or datetime value
Examples	hour( to_datetime('2012-07-22 13:24:57') ) → 13

Interval variant

Calculates the length in hours of an interval.

语法	hour(interval)
参数	interval - interval value to return number of hours from
Examples	hour(to_interval('3 hours')) → 3 hour(age('2012-07-22T13:00:00','2012-07-22T10:00:00')) → 3 hour(age('2012-01-01','2010-01-01')) → 17520

13.2.7.8. make_date

Creates a date value from year, month and day numbers.

语法	make_date(year, month, day)
参数	year - Year number. Years 1 to 99 are interpreted as is. Year 0 is invalid. month - Month number, where 1=January day - Day number, beginning with 1 for the first day in the month
Examples	make_date(2020,5,4) → date value 2020-05-04

13.2.7.9. make_datetime

Creates a datetime value from year, month, day, hour, minute and second numbers.

语法	make_datetime(year, month, day, hour, minute, second)
参数	year - Year number. Years 1 to 99 are interpreted as is. Year 0 is invalid. month - Month number, where 1=January day - Day number, beginning with 1 for the first day in the month hour - Hour number minute - Minutes second - Seconds (fractional values include milliseconds)
Examples	make_datetime(2020,5,4,13,45,30.5) → datetime value 2020-05-04 13:45:30.500

13.2.7.10. make_interval

Creates an interval value from year, month, weeks, days, hours, minute and seconds values.

语法	make_interval([years=0], [months=0], [weeks=0], [days=0], [hours=0], [minutes=0], [seconds=0]) [] 标记可选参数
参数	years - Number of years (assumes a 365.25 day year length). months - Number of months (assumes a 30 day month length) weeks - Number of weeks days - Number of days hours - Number of hours minutes - Number of minutes seconds - Number of seconds
Examples	make_interval(hours:=3) → interval: 3 hours make_interval(days:=2, hours:=3) → interval: 2.125 days make_interval(minutes:=0.5, seconds:=5) → interval: 35 seconds

13.2.7.11. make_time

Creates a time value from hour, minute and second numbers.

语法	make_time(hour, minute, second)
参数	hour - Hour number minute - Minutes second - Seconds (fractional values include milliseconds)
Examples	make_time(13,45,30.5) → time value 13:45:30.500

13.2.7.12. minute

Extracts the minutes part from a datetime or time, or the number of minutes from an interval.

Time variant

Extracts the minutes part from a time or datetime.

语法	minute(datetime)
参数	datetime - a time or datetime value
Examples	minute( to_datetime('2012-07-22 13:24:57') ) → 24

Interval variant

Calculates the length in minutes of an interval.

语法	minute(interval)
参数	interval - interval value to return number of minutes from
Examples	minute(to_interval('3 minutes')) → 3 minute(age('2012-07-22T00:20:00','2012-07-22T00:00:00')) → 20 minute(age('2012-01-01','2010-01-01')) → 1051200

13.2.7.13. month

Extracts the month part from a date, or the number of months from an interval.

Date variant

Extracts the month part from a date or datetime.

语法	month(date)
参数	date - a date or datetime value
Examples	month('2012-05-12') → 05

Interval variant

Calculates the length in months of an interval.

语法	month(interval)
参数	interval - interval value to return number of months from
Examples	month(to_interval('3 months')) → 3 month(age('2012-01-01','2010-01-01')) → 4.03333

13.2.7.14. now

Returns the current date and time. The function is static and will return consistent results while evaluating. The time returned is the time when the expression is prepared.

语法	now()
Examples	now() → 2012-07-22T13:24:57

13.2.7.15. second

Extracts the seconds part from a datetime or time, or the number of seconds from an interval.

Time variant

Extracts the seconds part from a time or datetime.

语法	second(datetime)
参数	datetime - a time or datetime value
Examples	second( to_datetime('2012-07-22 13:24:57') ) → 57

Interval variant

Calculates the length in seconds of an interval.

语法	second(interval)
参数	interval - interval value to return number of seconds from
Examples	second(to_interval('3 minutes')) → 180 second(age('2012-07-22T00:20:00','2012-07-22T00:00:00')) → 1200 second(age('2012-01-01','2010-01-01')) → 63072000

13.2.7.16. to_date

Converts a string into a date object. An optional format string can be provided to parse the string; see QDate::fromString or the documentation of the format_date function for additional documentation on the format. By default the current QGIS user locale is used.

语法	to_date(string, [format], [language]) [] 标记可选参数
参数	string - string representing a date value format - format used to convert the string into a date language - language (lowercase, two- or three-letter, ISO 639 language code) used to convert the string into a date. By default the current QGIS user locale is used.
Examples	to_date('2012-05-04') → 2012-05-04 to_date('June 29, 2019','MMMM d, yyyy') → 2019-06-29, if the current locale uses the name 'June' for the sixth month, otherwise an error occurs to_date('29 juin, 2019','d MMMM, yyyy','fr') → 2019-06-29

13.2.7.17. to_datetime

Converts a string into a datetime object. An optional format string can be provided to parse the string; see QDate::fromString, QTime::fromString or the documentation of the format_date function for additional documentation on the format. By default the current QGIS user locale is used.

语法	to_datetime(string, [format], [language]) [] 标记可选参数
参数	string - string representing a datetime value format - format used to convert the string into a datetime language - language (lowercase, two- or three-letter, ISO 639 language code) used to convert the string into a datetime. By default the current QGIS user locale is used.
Examples	to_datetime('2012-05-04 12:50:00') → 2012-05-04T12:50:00 to_datetime('June 29, 2019 @ 12:34','MMMM d, yyyy @ HH:mm') → 2019-06-29T12:34, if the current locale uses the name 'June' for the sixth month, otherwise an error occurs to_datetime('29 juin, 2019 @ 12:34','d MMMM, yyyy @ HH:mm','fr') → 2019-06-29T12:34

13.2.7.18. to_interval

Converts a string to an interval type. Can be used to take days, hours, month, etc of a date.

语法	to_interval(string)
参数	string - a string representing an interval. Allowable formats include {n} days {n} hours {n} months.
Examples	to_interval('1 day 2 hours') → interval: 1.08333 days to_interval( '0.5 hours' ) → interval: 30 minutes to_datetime('2012-05-05 12:00:00') - to_interval('1 day 2 hours') → 2012-05-04T10:00:00

13.2.7.19. to_time

Converts a string into a time object. An optional format string can be provided to parse the string; see QTime::fromString for additional documentation on the format.

语法	to_time(string, [format], [language]) [] 标记可选参数
参数	string - string representing a time value format - format used to convert the string into a time language - language (lowercase, two- or three-letter, ISO 639 language code) used to convert the string into a time
Examples	to_time('12:30:01') → 12:30:01 to_time('12:34','HH:mm') → 12:34:00 to_time('12:34','HH:mm','fr') → 12:34:00

13.2.7.20. week

Extracts the week number from a date, or the number of weeks from an interval.

Date variant

Extracts the week number from a date or datetime.

语法	week(date)
参数	date - a date or datetime value
Examples	week('2012-05-12') → 19

Interval variant

Calculates the length in weeks of an interval.

语法	week(interval)
参数	interval - interval value to return number of months from
Examples	week(to_interval('3 weeks')) → 3 week(age('2012-01-01','2010-01-01')) → 104.285

13.2.7.21. year

Extracts the year part from a date, or the number of years from an interval.

Date variant

Extracts the year part from a date or datetime.

语法	year(date)
参数	date - a date or datetime value
Examples	year('2012-05-12') → 2012

Interval variant

Calculates the length in years of an interval.

语法	year(interval)
参数	interval - interval value to return number of years from
Examples	year(to_interval('3 years')) → 3 year(age('2012-01-01','2010-01-01')) → 1.9986

Some examples:

Besides these functions, subtracting dates, datetimes or times using the - (minus) operator will return an interval.

Adding or subtracting an interval to dates, datetimes or times, using the + (plus) and - (minus) operators, will return a datetime.

• Get the number of days until QGIS 3.0 release:

  to_date('2017-09-29') - to_date(now())
  -- Returns <interval: 203 days>
  

• The same with time:

  to_datetime('2017-09-29 12:00:00') - now()
  -- Returns <interval: 202.49 days>
  

• Get the datetime of 100 days from now:

  now() + to_interval('100 days')
  -- Returns <datetime: 2017-06-18 01:00:00>
  

13.2.8. Fields and Values

Contains a list of fields from the active layer, and special values. Fields list includes the ones stored in the dataset, virtual and auxiliary ones as well as from joins.

Double-click a field name to have it added to your expression. You can also type the field name (preferably inside double quotes) or its alias.

To retrieve fields values to use in an expression, select the appropriate field and, in the shown widget, choose between 10 Samples and All Unique. Requested values are then displayed and you can use the Search box at the top of the list to filter the result. Sample values can also be accessed via right-clicking on a field.

To add a value to the expression you are writing, double-click on it in the list. If the value is of a string type, it should be simple quoted, otherwise no quote is needed.

13.2.8.1. NULL

Equates to a NULL value.

语法	NULL
Examples	NULL → a NULL value

备注

To test for NULL use an IS NULL or IS NOT NULL expression.

13.2.9. Files and Paths Functions

This group contains functions which manipulate file and path names.

13.2.9.1. base_file_name

Returns the base name of the file without the directory or file suffix.

语法	base_file_name(path)
参数	path - a file path or a map layer value. If a map layer value is specified then the file source of the layer will be used.
Examples	base_file_name('/home/qgis/data/country_boundaries.shp') → 'country_boundaries'

13.2.9.2. exif

Retrieves exif tag values from an image file.

语法	exif(path, [tag]) [] 标记可选参数
参数	path - An image file path or a map layer value. If a map layer value is specified then the file source of the layer will be used. tag - The tag to return. If empty, a map with all exif tag values will be returned.
Examples	exif('/my/photo.jpg','Exif.Image.Orientation') → 0

13.2.9.3. file_exists

Returns TRUE if a file path exists.

语法	file_exists(path)
参数	path - a file path or a map layer value. If a map layer value is specified then the file source of the layer will be used.
Examples	file_exists('/home/qgis/data/country_boundaries.shp') → TRUE

13.2.9.4. file_name

Returns the name of a file (including the file extension), excluding the directory.

语法	file_name(path)
参数	path - a file path or a map layer value. If a map layer value is specified then the file source of the layer will be used.
Examples	file_name('/home/qgis/data/country_boundaries.shp') → 'country_boundaries.shp'

13.2.9.5. file_path

Returns the directory component of a file path. This does not include the file name.

语法	file_path(path)
参数	path - a file path or a map layer value. If a map layer value is specified then the file source of the layer will be used.
Examples	file_path('/home/qgis/data/country_boundaries.shp') → '/home/qgis/data'

13.2.9.6. file_size

Returns the size (in bytes) of a file.

语法	file_size(path)
参数	path - a file path or a map layer value. If a map layer value is specified then the file source of the layer will be used.
Examples	file_size('/home/qgis/data/country_boundaries.geojson') → 5674

13.2.9.7. file_suffix

Returns the file suffix (extension) from a file path.

语法	file_suffix(path)
参数	path - a file path or a map layer value. If a map layer value is specified then the file source of the layer will be used.
Examples	file_suffix('/home/qgis/data/country_boundaries.shp') → 'shp'

13.2.9.8. is_directory

Returns TRUE if a path corresponds to a directory.

语法	is_directory(path)
参数	path - a file path or a map layer value. If a map layer value is specified then the file source of the layer will be used.
Examples	is_directory('/home/qgis/data/country_boundaries.shp') → FALSE is_directory('/home/qgis/data/') → TRUE

13.2.9.9. is_file

Returns TRUE if a path corresponds to a file.

语法	is_file(path)
参数	path - a file path or a map layer value. If a map layer value is specified then the file source of the layer will be used.
Examples	is_file('/home/qgis/data/country_boundaries.shp') → TRUE is_file('/home/qgis/data/') → FALSE

13.2.10. Form Functions

This group contains functions that operate exclusively under the attribute form context. For example, in field's widgets settings.

13.2.10.1. current_parent_value

Only usable in an embedded form context, this function returns the current, unsaved value of a field in the parent form currently being edited. This will differ from the parent feature's actual attribute values for features which are currently being edited or have not yet been added to a parent layer. When used in a value-relation widget filter expression, this function should be wrapped into a 'coalesce()' that can retrieve the actual parent feature from the layer when the form is not used in an embedded context.

语法	current_parent_value(field_name)
参数	field_name - a field name in the current parent form
Examples	current_parent_value( 'FIELD_NAME' ) → The current value of a field 'FIELD_NAME' in the parent form.

13.2.10.2. current_value

Returns the current, unsaved value of a field in the form or table row currently being edited. This will differ from the feature's actual attribute values for features which are currently being edited or have not yet been added to a layer.

语法	current_value(field_name)
参数	field_name - a field name in the current form or table row
Examples	current_value( 'FIELD_NAME' ) → The current value of field 'FIELD_NAME'.

13.2.11. Fuzzy Matching Functions

This group contains functions for fuzzy comparisons between values.

13.2.11.1. hamming_distance

Returns the Hamming distance between two strings. This equates to the number of characters at corresponding positions within the input strings where the characters are different. The input strings must be the same length, and the comparison is case-sensitive.

语法	hamming_distance(string1, string2)
参数	string1 - a string string2 - a string
Examples	hamming_distance('abc','xec') → 2 hamming_distance('abc','ABc') → 2 hamming_distance(upper('abc'),upper('ABC')) → 0 hamming_distance('abc','abcd') → NULL

13.2.11.2. levenshtein

Returns the Levenshtein edit distance between two strings. This equates to the minimum number of character edits (insertions, deletions or substitutions) required to change one string to another.

The Levenshtein distance is a measure of the similarity between two strings. Smaller distances mean the strings are more similar, and larger distances indicate more different strings. The distance is case sensitive.

语法	levenshtein(string1, string2)
参数	string1 - a string string2 - a string
Examples	levenshtein('kittens','mitten') → 2 levenshtein('Kitten','kitten') → 1 levenshtein(upper('Kitten'),upper('kitten')) → 0

13.2.11.3. longest_common_substring

Returns the longest common substring between two strings. This substring is the longest string that is a substring of the two input strings. For example, the longest common substring of "ABABC" and "BABCA" is "BABC". The substring is case sensitive.

语法	longest_common_substring(string1, string2)
参数	string1 - a string string2 - a string
Examples	longest_common_substring('ABABC','BABCA') → 'BABC' longest_common_substring('abcDeF','abcdef') → 'abc' longest_common_substring(upper('abcDeF'),upper('abcdex')) → 'ABCDE'

13.2.11.4. soundex

Returns the Soundex representation of a string. Soundex is a phonetic matching algorithm, so strings with similar sounds should be represented by the same Soundex code.

语法	soundex(string)
参数	string - a string
Examples	soundex('robert') → 'R163' soundex('rupert') → 'R163' soundex('rubin') → 'R150'

13.2.12. General Functions

This group contains general assorted functions.

13.2.12.1. env

Gets an environment variable and returns its content as a string. If the variable is not found, NULL will be returned. This is handy to inject system specific configuration like drive letters or path prefixes. Definition of environment variables depends on the operating system, please check with your system administrator or the operating system documentation how this can be set.

语法	env(name)
参数	name - The name of the environment variable which should be retrieved.
Examples	env( 'LANG' ) → 'en_US.UTF-8' env( 'MY_OWN_PREFIX_VAR' ) → 'Z:' env( 'I_DO_NOT_EXIST' ) → NULL

13.2.12.2. eval

Evaluates an expression which is passed in a string. Useful to expand dynamic parameters passed as context variables or fields.

语法	eval(expression)
参数	expression - an expression string
Examples	eval('\'nice\'') → 'nice' eval(@expression_var) → [whatever the result of evaluating @expression_var might be…]

13.2.12.3. eval_template

Evaluates a template which is passed in a string. Useful to expand dynamic parameters passed as context variables or fields.

语法	eval_template(template)
参数	template - a template string
Examples	eval_template('QGIS [% upper(\'rocks\') %]') → QGIS ROCKS

13.2.12.4. is_layer_visible

Returns TRUE if a specified layer is visible.

语法	is_layer_visible(layer)
参数	图层 - 字符串, 表示图层名称或图层ID
Examples	is_layer_visible('baseraster') → TRUE

13.2.12.5. mime_type

Returns the mime type of the binary data.

语法	mime_type(bytes)
参数	bytes - the binary data
Examples	mime_type('<html><body></body></html>') → text/html mime_type(from_base64('R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAIAOw==')) → image/gif

13.2.12.6. var

Returns the value stored within a specified variable.

语法	var(name)
参数	name - a variable name
Examples	var('qgis_version') → '2.12'

Further reading: List of default variables

13.2.12.7. with_variable

This function sets a variable for any expression code that will be provided as 3rd argument. This is only useful for complicated expressions, where the same calculated value needs to be used in different places.

语法	with_variable(name, value, expression)
参数	name - the name of the variable to set value - the value to set expression - the expression for which the variable will be available
Examples	with_variable('my_sum', 1 + 2 + 3, @my_sum * 2 + @my_sum * 5) → 42

13.2.13. Geometry Functions

This group contains functions that operate on geometry objects (e.g. buffer, transform, $area).

13.2.13.1. affine_transform

Returns the geometry after an affine transformation. Calculations are in the Spatial Reference System of this geometry. The operations are performed in a scale, rotation, translation order. If there is a Z or M offset but the coordinate is not present in the geometry, it will be added.

语法	affine_transform(geometry, delta_x, delta_y, rotation_z, scale_x, scale_y, [delta_z=0], [delta_m=0], [scale_z=1], [scale_m=1]) [] 标记可选参数
参数	geometry - a geometry delta_x - x-axis translation delta_y - y-axis translation rotation_z - rotation around z-axis in degrees counter-clockwise scale_x - x-axis scale factor scale_y - y-axis scale factor delta_z - z-axis translation delta_m - m-axis translation scale_z - z-axis scale factor scale_m - m-axis scale factor
Examples	geom_to_wkt(affine_transform(geom_from_wkt('LINESTRING(1 1, 2 2)'), 2, 2, 0, 1, 1)) → 'LineString (3 3, 4 4)' geom_to_wkt(affine_transform(geom_from_wkt('POLYGON((0 0, 0 3, 2 2, 0 0))'), 0, 0, -90, 1, 2)) → 'Polygon ((0 0, 6 0, 4 -2, 0 0))' geom_to_wkt(affine_transform(geom_from_wkt('POINT(3 1)'), 0, 0, 0, 1, 1, 5, 0)) → 'PointZ (3 1 5)'

13.2.13.2. angle_at_vertex

Returns the bisector angle (average angle) to the geometry for a specified vertex on a linestring geometry. Angles are in degrees clockwise from north.

语法	angle_at_vertex(geometry, vertex)
参数	geometry - a linestring geometry vertex - vertex index, starting from 0; if the value is negative, the selected vertex index will be its total count minus the absolute value
Examples	angle_at_vertex(geometry:=geom_from_wkt('LineString(0 0, 10 0, 10 10)'),vertex:=1) → 45.0

13.2.13.3. apply_dash_pattern

Applies a dash pattern to a geometry, returning a MultiLineString geometry which is the input geometry stroked along each line/ring with the specified pattern.

语法	apply_dash_pattern(geometry, pattern, [start_rule=no_rule], [end_rule=no_rule], [adjustment=both], [pattern_offset=0]) [] 标记可选参数
参数	geometry - a geometry (accepts (multi)linestrings or (multi)polygons). pattern - dash pattern, as an array of numbers representing dash and gap lengths. Must contain an even number of elements. start_rule - optional rule for constraining the start of the pattern. Valid values are 'no_rule', 'full_dash', 'half_dash', 'full_gap', 'half_gap'. end_rule - optional rule for constraining the end of the pattern. Valid values are 'no_rule', 'full_dash', 'half_dash', 'full_gap', 'half_gap'. adjustment - optional rule for specifying which part of patterns are adjusted to fit the desired pattern rules. Valid values are 'both', 'dash', 'gap'. pattern_offset - Optional distance specifying a specific distance along the pattern to commence at.
Examples	geom_to_wkt(apply_dash_pattern(geom_from_wkt('LINESTRING(1 1, 10 1)'), array(3, 1))) → MultiLineString ((1 1, 4 1),(5 1, 8 1),(9 1, 10 1, 10 1)) geom_to_wkt(apply_dash_pattern(geom_from_wkt('LINESTRING(1 1, 10 1)'), array(3, 1), start_rule:='half_dash')) → MultiLineString ((1 1, 2.5 1),(3.5 1, 6.5 1),(7.5 1, 10 1, 10 1))

13.2.13.4. $area

Returns the area of the current feature. The area calculated by this function respects both the current project's ellipsoid setting and area unit settings. For example, if an ellipsoid has been set for the project then the calculated area will be ellipsoidal, and if no ellipsoid is set then the calculated area will be planimetric.

语法	$area
Examples	$area → 42

13.2.13.5. area

Returns the area of a geometry polygon object. Calculations are always planimetric in the Spatial Reference System (SRS) of this geometry, and the units of the returned area will match the units for the SRS. This differs from the calculations performed by the $area function, which will perform ellipsoidal calculations based on the project's ellipsoid and area unit settings.

语法	area(geometry)
参数	geometry - polygon geometry object
Examples	area(geom_from_wkt('POLYGON((0 0, 4 0, 4 2, 0 2, 0 0))')) → 8.0

13.2.13.6. azimuth

Returns the north-based azimuth as the angle in radians measured clockwise from the vertical on point_a to point_b.

语法	azimuth(point_a, point_b)
参数	point_a - point geometry point_b - point geometry
Examples	degrees( azimuth( make_point(25, 45), make_point(75, 100) ) ) → 42.273689 degrees( azimuth( make_point(75, 100), make_point(25,45) ) ) → 222.273689

13.2.13.7. boundary

Returns the closure of the combinatorial boundary of the geometry (ie the topological boundary of the geometry). For instance, a polygon geometry will have a boundary consisting of the linestrings for each ring in the polygon. Some geometry types do not have a defined boundary, e.g., points or geometry collections, and will return NULL.

语法	boundary(geometry)
参数	geometry - a geometry
Examples	geom_to_wkt(boundary(geom_from_wkt('Polygon((1 1, 0 0, -1 1, 1 1))'))) → 'LineString(1 1,0 0,-1 1,1 1)' geom_to_wkt(boundary(geom_from_wkt('LineString(1 1,0 0,-1 1)'))) → 'MultiPoint ((1 1),(-1 1))'

Further reading: Boundary algorithm

13.2.13.8. bounds

Returns a geometry which represents the bounding box of an input geometry. Calculations are in the Spatial Reference System of this geometry.

语法	bounds(geometry)
参数	geometry - a geometry
Examples	bounds($geometry) → bounding box of the current feature's geometry geom_to_wkt(bounds(geom_from_wkt('Polygon((1 1, 0 0, -1 1, 1 1))'))) → 'Polygon ((-1 0, 1 0, 1 1, -1 1, -1 0))'

图 13.4 Black lines represent the bounding boxes of each polygon feature

Further reading: Bounding boxes algorithm

13.2.13.9. bounds_height

Returns the height of the bounding box of a geometry. Calculations are in the Spatial Reference System of this geometry.

语法	bounds_height(geometry)
参数	geometry - a geometry
Examples	bounds_height($geometry) → height of bounding box of the current feature's geometry bounds_height(geom_from_wkt('Polygon((1 1, 0 0, -1 1, 1 1))')) → 1

13.2.13.10. bounds_width

Returns the width of the bounding box of a geometry. Calculations are in the Spatial Reference System of this geometry.

语法	bounds_width(geometry)
参数	geometry - a geometry
Examples	bounds_width($geometry) → width of bounding box of the current feature's geometry bounds_width(geom_from_wkt('Polygon((1 1, 0 0, -1 1, 1 1))')) → 2

13.2.13.11. buffer

Returns a geometry that represents all points whose distance from this geometry is less than or equal to distance. Calculations are in the Spatial Reference System of this geometry.

语法	buffer(geometry, distance, [segments=8], [cap='round'], [join='round'], [miter_limit=2]) [] 标记可选参数
参数	geometry - a geometry distance - buffer distance in layer units segments - number of segments to use to represent a quarter circle when a round join style is used. A larger number results in a smoother buffer with more nodes. cap - end cap style for buffer. Valid values are 'round', 'flat' or 'square' join - join style for buffer. Valid values are 'round', 'bevel' or 'miter'. miter_limit - miter distance limit, for use when the join style is set to 'miter'
Examples	buffer($geometry, 10.5) → polygon of the current feature's geometry buffered by 10.5 units

图 13.5 Buffer (in yellow) of points, line, polygon with positive buffer, and polygon with negative buffer

Further reading: Buffer algorithm

13.2.13.12. buffer_by_m

Creates a buffer along a line geometry where the buffer diameter varies according to the m-values at the line vertices.

语法	buffer_by_m(geometry, [segments=8]) [] 标记可选参数
参数	geometry - input geometry. Must be a (multi)line geometry with m values. segments - number of segments to approximate quarter-circle curves in the buffer.
Examples	buffer_by_m(geometry:=geom_from_wkt('LINESTRINGM(1 2 0.5, 4 2 0.2)'),segments:=8) → A variable width buffer starting with a diameter of 0.5 and ending with a diameter of 0.2 along the linestring geometry.

图 13.6 Buffering line features using the m value on the vertices

Further reading: Variable width buffer (by M value) algorithm

13.2.13.13. centroid

Returns the geometric center of a geometry.

语法	centroid(geometry)
参数	geometry - a geometry
Examples	centroid($geometry) → a point geometry

Further reading: Centroids algorithm

13.2.13.14. close_line

Returns a closed line string of the input line string by appending the first point to the end of the line, if it is not already closed. If the geometry is not a line string or multi line string then the result will be NULL.

语法	close_line(geometry)
参数	geometry - a line string geometry
Examples	geom_to_wkt(close_line(geom_from_wkt('LINESTRING(0 0, 1 0, 1 1)'))) → 'LineString (0 0, 1 0, 1 1, 0 0)' geom_to_wkt(close_line(geom_from_wkt('LINESTRING(0 0, 1 0, 1 1, 0 0)'))) → 'LineString (0 0, 1 0, 1 1, 0 0)'

13.2.13.15. closest_point

Returns the point on geometry1 that is closest to geometry2.

语法	closest_point(geometry1, geometry2)
参数	geometry1 - geometry to find closest point on geometry2 - geometry to find closest point to
Examples	geom_to_wkt(closest_point(geom_from_wkt('LINESTRING (20 80, 98 190, 110 180, 50 75 )'),geom_from_wkt('POINT(100 100)'))) → 'Point(73.0769 115.384)'

13.2.13.16. collect_geometries

Collects a set of geometries into a multi-part geometry object.

List of arguments variant

Geometry parts are specified as separate arguments to the function.

语法	collect_geometries(geometry1, geometry2, ...)
参数	geometry - a geometry
Examples	geom_to_wkt(collect_geometries(make_point(1,2), make_point(3,4), make_point(5,6))) → 'MultiPoint ((1 2),(3 4),(5 6))'

Array variant

Geometry parts are specified as an array of geometry parts.

语法	collect_geometries(array)
参数	array - array of geometry objects
Examples	geom_to_wkt(collect_geometries(array(make_point(1,2), make_point(3,4), make_point(5,6)))) → 'MultiPoint ((1 2),(3 4),(5 6))'

Further reading: Collect geometries algorithm

13.2.13.17. combine

Returns the combination of two geometries.

语法	combine(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	geom_to_wkt( combine( geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ), geom_from_wkt( 'LINESTRING(3 3, 4 4, 2 1)' ) ) ) → 'MULTILINESTRING((4 4, 2 1), (3 3, 4 4), (4 4, 5 5))' geom_to_wkt( combine( geom_from_wkt( 'LINESTRING(3 3, 4 4)' ), geom_from_wkt( 'LINESTRING(3 3, 6 6, 2 1)' ) ) ) → 'LINESTRING(3 3, 4 4, 6 6, 2 1)'

13.2.13.18. concave_hull

Returns a possibly concave polygon that contains all the points in the geometry

语法	concave_hull(geometry, target_percent, [allow_holes=False]) [] 标记可选参数
参数	geometry - a geometry target_percent - the percentage of area of the convex hull the solution tries to approach. A target_percent of 1 gives the same result as the convex hull. A target_percent between 0 and 0.99 produces a result that should have a smaller area than the convex hull. allow_holes - optional argument specifying whether to allow holes within the output geometry. Defaults to FALSE, set to TRUE to avoid including holes in the output geometry.
Examples	geom_to_wkt(concave_hull(geom_from_wkt('MULTILINESTRING((106 164,30 112,74 70,82 112,130 94,130 62,122 40,156 32,162 76,172 88),(132 178,134 148,128 136,96 128,132 108,150 130,170 142,174 110,156 96,158 90,158 88),(22 64,66 28,94 38,94 68,114 76,112 30,132 10,168 18,178 34,186 52,184 74,190 100,190 122,182 148,178 170,176 184,156 164,146 178,132 186,92 182,56 158,36 150,62 150,76 128,88 118))'), 0.99)) → 'Polygon ((30 112, 36 150, 92 182, 132 186, 176 184, 190 122, 190 100, 186 52, 178 34, 168 18, 132 10, 112 30, 66 28, 22 64, 30 112))'

Further reading: convex_hull

13.2.13.19. contains

Tests whether a geometry contains another. Returns TRUE if and only if no points of geometry2 lie in the exterior of geometry1, and at least one point of the interior of geometry2 lies in the interior of geometry1.

语法	contains(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	contains( geom_from_wkt( 'POLYGON((0 0, 0 1, 1 1, 1 0, 0 0))' ), geom_from_wkt( 'POINT(0.5 0.5 )' ) ) → TRUE contains( geom_from_wkt( 'POLYGON((0 0, 0 1, 1 1, 1 0, 0 0))' ), geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ) ) → FALSE

Further reading: overlay_contains

13.2.13.20. convex_hull

Returns the convex hull of a geometry. It represents the minimum convex geometry that encloses all geometries within the set.

语法	convex_hull(geometry)
参数	geometry - a geometry
Examples	geom_to_wkt( convex_hull( geom_from_wkt( 'LINESTRING(3 3, 4 4, 4 10)' ) ) ) → 'POLYGON((3 3, 4 10, 4 4, 3 3))'

Further reading: concave_hull, Convex hull algorithm

13.2.13.21. crosses

Tests whether a geometry crosses another. Returns TRUE if the supplied geometries have some, but not all, interior points in common.

语法	crosses(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	crosses( geom_from_wkt( 'LINESTRING(3 5, 4 4, 5 3)' ), geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ) ) → TRUE crosses( geom_from_wkt( 'POINT(4 5)' ), geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ) ) → FALSE

Further reading: overlay_crosses

13.2.13.22. densify_by_count

Takes a polygon or line layer geometry and generates a new one in which the geometries have a larger number of vertices than the original one.

语法	densify_by_count(geometry, vertices)
参数	geometry - a geometry (accepts (multi)linestrings or (multi)polygons). vertices - number of vertices to add (per segment)
Examples	geom_to_wkt(densify_by_count(geom_from_wkt('LINESTRING(1 1, 10 1)'), 3)) → LineString (1 1, 3.25 1, 5.5 1, 7.75 1, 10 1)

Further reading: Densify by count algorithm

13.2.13.23. densify_by_distance

Takes a polygon or line layer geometry and generates a new one in which the geometries are densified by adding additional vertices on edges that have a maximum distance of the specified interval distance.

语法	densify_by_distance(geometry, distance)
参数	geometry - a geometry (accepts (multi)linestrings or (multi)polygons). distance - maximum interval distance between vertices in output geometry
Examples	geom_to_wkt(densify_by_distance(geom_from_wkt('LINESTRING(1 1, 10 1)'), 4)) → LineString (1 1, 4 1, 7 1, 10 1)

Further reading: Densify by interval algorithm

13.2.13.24. difference

Returns a geometry that represents that part of geometry1 that does not intersect with geometry2.

语法	difference(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	geom_to_wkt( difference( geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ), geom_from_wkt( 'LINESTRING(3 3, 4 4)' ) ) ) → 'LINESTRING(4 4, 5 5)'

Further reading: Difference algorithm

13.2.13.25. disjoint

Tests whether geometries do not spatially intersect. Returns TRUE if the geometries do not share any space together.

语法	disjoint(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	disjoint( geom_from_wkt( 'POLYGON((0 0, 0 1, 1 1, 1 0, 0 0 ))' ), geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ) ) → TRUE disjoint( geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ), geom_from_wkt( 'POINT(4 4)' )) → FALSE

Further reading: overlay_disjoint

13.2.13.26. distance

Returns the minimum distance (based on spatial reference) between two geometries in projected units.

语法	distance(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	distance( geom_from_wkt( 'POINT(4 4)' ), geom_from_wkt( 'POINT(4 8)' ) ) → 4

13.2.13.27. distance_to_vertex

Returns the distance along the geometry to a specified vertex.

语法	distance_to_vertex(geometry, vertex)
参数	geometry - a linestring geometry vertex - vertex index, starting from 0; if the value is negative, the selected vertex index will be its total count minus the absolute value
Examples	distance_to_vertex(geometry:=geom_from_wkt('LineString(0 0, 10 0, 10 10)'),vertex:=1) → 10.0

13.2.13.28. end_point

Returns the last node from a geometry.

语法	end_point(geometry)
参数	geometry - geometry object
Examples	geom_to_wkt(end_point(geom_from_wkt('LINESTRING(4 0, 4 2, 0 2)'))) → 'Point (0 2)'

Further reading: Extract specific vertices algorithm

13.2.13.29. exif_geotag

Creates a point geometry from the exif geotags of an image file.

语法	exif_geotag(path)
参数	path - An image file path or a map layer value. If a map layer value is specified then the file source of the layer will be used.
Examples	geom_to_wkt(exif_geotag('/my/photo.jpg')) → 'Point (2 4)'

13.2.13.30. extend

Extends the start and end of a linestring geometry by a specified amount. Lines are extended using the bearing of the first and last segment in the line. For a multilinestring, all the parts are extended. Distances are in the Spatial Reference System of this geometry.

语法	extend(geometry, start_distance, end_distance)
参数	geometry - a (multi)linestring geometry start_distance - distance to extend the start of the line end_distance - distance to extend the end of the line.
Examples	geom_to_wkt(extend(geom_from_wkt('LineString(0 0, 1 0, 1 1)'),1,2)) → 'LineString (-1 0, 1 0, 1 3)' geom_to_wkt(extend(geom_from_wkt('MultiLineString((0 0, 1 0, 1 1), (2 2, 0 2, 0 5))'),1,2)) → 'MultiLineString ((-1 0, 1 0, 1 3),(3 2, 0 2, 0 7))'

Further reading: Extend lines algorithm

13.2.13.31. exterior_ring

Returns a line string representing the exterior ring of a polygon geometry. If the geometry is not a polygon then the result will be NULL.

语法	exterior_ring(geometry)
参数	geometry - a polygon geometry
Examples	geom_to_wkt(exterior_ring(geom_from_wkt('POLYGON((-1 -1, 4 0, 4 2, 0 2, -1 -1),( 0.1 0.1, 0.1 0.2, 0.2 0.2, 0.2, 0.1, 0.1 0.1))'))) → 'LineString (-1 -1, 4 0, 4 2, 0 2, -1 -1)'

13.2.13.32. extrude

Returns an extruded version of the input (Multi-)Curve or (Multi-)Linestring geometry with an extension specified by x and y.

语法	extrude(geometry, x, y)
参数	geometry - a curve or linestring geometry x - x extension, numeric value y - y extension, numeric value
Examples	geom_to_wkt(extrude(geom_from_wkt('LineString(1 2, 3 2, 4 3)'), 1, 2)) → 'Polygon ((1 2, 3 2, 4 3, 5 5, 4 4, 2 4, 1 2))' geom_to_wkt(extrude(geom_from_wkt('MultiLineString((1 2, 3 2), (4 3, 8 3))'), 1, 2)) → 'MultiPolygon (((1 2, 3 2, 4 4, 2 4, 1 2)),((4 3, 8 3, 9 5, 5 5, 4 3)))'

13.2.13.33. flip_coordinates

Returns a copy of the geometry with the x and y coordinates swapped. Useful for repairing geometries which have had their latitude and longitude values reversed.

语法	flip_coordinates(geometry)
参数	geometry - a geometry
Examples	geom_to_wkt(flip_coordinates(make_point(1, 2))) → 'Point (2 1)' geom_to_wkt(flip_coordinates(geom_from_wkt('LineString(0 2, 1 0, 1 6)'))) → 'LineString (2 0, 0 1, 6 1)'

Further reading: Swap X and Y coordinates algorithm

13.2.13.34. force_polygon_ccw

Forces a geometry to respect the convention where exterior rings are counter-clockwise, interior rings are clockwise.

语法	force_polygon_ccw(geometry)
参数	geometry - a geometry. Any non-polygon geometries are returned unchanged.
Examples	geom_to_wkt(force_polygon_ccw(geometry:=geom_from_wkt('Polygon ((-1 -1, 0 2, 4 2, 4 0, -1 -1))'))) → 'Polygon ((-1 -1, 4 0, 4 2, 0 2, -1 -1))'

Further reading: force_polygon_cw, force_rhr

13.2.13.35. force_polygon_cw

Forces a geometry to respect the convention where exterior rings are clockwise, interior rings are counter-clockwise.

语法	force_polygon_cw(geometry)
参数	geometry - a geometry. Any non-polygon geometries are returned unchanged.
Examples	geom_to_wkt(force_polygon_cw(geometry:=geom_from_wkt('POLYGON((-1 -1, 4 0, 4 2, 0 2, -1 -1))'))) → 'Polygon ((-1 -1, 0 2, 4 2, 4 0, -1 -1))'

Further reading: force_polygon_ccw, force_rhr

13.2.13.36. force_rhr

Forces a geometry to respect the Right-Hand-Rule, in which the area that is bounded by a polygon is to the right of the boundary. In particular, the exterior ring is oriented in a clockwise direction and the interior rings in a counter-clockwise direction. Due to the inconsistency in the definition of the Right-Hand-Rule in some contexts it is recommended to use the explicit force_polygon_cw function instead.

语法	force_rhr(geometry)
参数	geometry - a geometry. Any non-polygon geometries are returned unchanged.
Examples	geom_to_wkt(force_rhr(geometry:=geom_from_wkt('POLYGON((-1 -1, 4 0, 4 2, 0 2, -1 -1))'))) → 'Polygon ((-1 -1, 0 2, 4 2, 4 0, -1 -1))'

Further reading: Force right-hand-rule algorithm, force_polygon_ccw, force_polygon_cw

13.2.13.37. geom_from_gml

Returns a geometry from a GML representation of geometry.

语法	geom_from_gml(gml)
参数	gml - GML representation of a geometry as a string
Examples	geom_from_gml('<gml:LineString srsName="EPSG:4326"><gml:coordinates>4,4 5,5 6,6</gml:coordinates></gml:LineString>') → a line geometry object

13.2.13.38. geom_from_wkb

Returns a geometry created from a Well-Known Binary (WKB) representation.

语法	geom_from_wkb(binary)
参数	binary - Well-Known Binary (WKB) representation of a geometry (as a binary blob)
Examples	geom_from_wkb( geom_to_wkb( make_point(4,5) ) ) → a point geometry object

13.2.13.39. geom_from_wkt

Returns a geometry created from a Well-Known Text (WKT) representation.

语法	geom_from_wkt(text)
参数	text - Well-Known Text (WKT) representation of a geometry
Examples	geom_from_wkt( 'POINT(4 5)' ) → a geometry object

13.2.13.40. geom_to_wkb

Returns the Well-Known Binary (WKB) representation of a geometry

语法	geom_to_wkb(geometry)
参数	geometry - a geometry
Examples	geom_to_wkb( $geometry ) → binary blob containing a geometry object

13.2.13.41. geom_to_wkt

Returns the Well-Known Text (WKT) representation of the geometry without SRID metadata.

语法	geom_to_wkt(geometry, [precision=8]) [] 标记可选参数
参数	geometry - a geometry precision - numeric precision
Examples	geom_to_wkt( make_point(6, 50) ) → 'POINT(6 50)' geom_to_wkt(centroid(geom_from_wkt('Polygon((1 1, 0 0, -1 1, 1 1))'))) → 'POINT(0 0.66666667)' geom_to_wkt(centroid(geom_from_wkt('Polygon((1 1, 0 0, -1 1, 1 1))')), 2) → 'POINT(0 0.67)'

13.2.13.42. $geometry

Returns the geometry of the current feature. Can be used for processing with other functions. WARNING: This function is deprecated. It is recommended to use the replacement @geometry variable instead.

语法	$geometry
Examples	geom_to_wkt( $geometry ) → 'POINT(6 50)'

13.2.13.43. geometry

Returns a feature's geometry.

语法	geometry(feature)
参数	feature - a feature object
Examples	`` geometry( $currentfeature )`` → the geometry of the current feature. Prefer using $geometry. geom_to_wkt( geometry( get_feature_by_id( 'streets', 1 ) ) ) → the geometry in WKT of the feature with the id 1 on the layer "streets", e.g. 'POINT(6 50)' intersects( $geometry, geometry( get_feature( 'streets', 'name', 'Main St.' ) ) ) → TRUE if the current feature spatially intersects the 'Main St.' named feature in the "streets" layer

13.2.13.44. geometry_n

Returns a specific geometry from a geometry collection, or NULL if the input geometry is not a collection. Also returns a part from a multipart geometry.

语法	geometry_n(geometry, index)
参数	geometry - geometry collection index - index of geometry to return, where 1 is the first geometry in the collection
Examples	geom_to_wkt(geometry_n(geom_from_wkt('GEOMETRYCOLLECTION(POINT(0 1), POINT(0 0), POINT(1 0), POINT(1 1))'),3)) → 'Point (1 0)'

13.2.13.45. geometry_type

Returns a string value describing the type of a geometry (Point, Line or Polygon)

语法	geometry_type(geometry)
参数	geometry - a geometry
Examples	geometry_type( geom_from_wkt( 'LINESTRING(2 5, 3 6, 4 8)') ) → 'Line' geometry_type( geom_from_wkt( 'MULTILINESTRING((2 5, 3 6, 4 8), (1 1, 0 0))') ) → 'Line' geometry_type( geom_from_wkt( 'POINT(2 5)') ) → 'Point' geometry_type( geom_from_wkt( 'POLYGON((-1 -1, 4 0, 4 2, 0 2, -1 -1))') ) → 'Polygon'

13.2.13.46. hausdorff_distance

Returns the Hausdorff distance between two geometries. This is basically a measure of how similar or dissimilar 2 geometries are, with a lower distance indicating more similar geometries.

The function can be executed with an optional densify fraction argument. If not specified, an approximation to the standard Hausdorff distance is used. This approximation is exact or close enough for a large subset of useful cases. Examples of these are:

• computing distance between Linestrings that are roughly parallel to each other, and roughly equal in length. This occurs in matching linear networks.

• Testing similarity of geometries.

If the default approximate provided by this method is insufficient, specify the optional densify fraction argument. Specifying this argument performs a segment densification before computing the discrete Hausdorff distance. The parameter sets the fraction by which to densify each segment. Each segment will be split into a number of equal-length subsegments, whose fraction of the total length is closest to the given fraction. Decreasing the densify fraction parameter will make the distance returned approach the true Hausdorff distance for the geometries.

语法	hausdorff_distance(geometry1, geometry2, [densify_fraction]) [] 标记可选参数
参数	geometry1 - a geometry geometry2 - a geometry densify_fraction - densify fraction amount
Examples	hausdorff_distance( geometry1:= geom_from_wkt('LINESTRING (0 0, 2 1)'),geometry2:=geom_from_wkt('LINESTRING (0 0, 2 0)')) → 2 hausdorff_distance( geom_from_wkt('LINESTRING (130 0, 0 0, 0 150)'),geom_from_wkt('LINESTRING (10 10, 10 150, 130 10)')) → 14.142135623 hausdorff_distance( geom_from_wkt('LINESTRING (130 0, 0 0, 0 150)'),geom_from_wkt('LINESTRING (10 10, 10 150, 130 10)'),0.5) → 70.0

13.2.13.47. inclination

Returns the inclination measured from the zenith (0) to the nadir (180) on point_a to point_b.

语法	inclination(point_a, point_b)
参数	point_a - point geometry point_b - point geometry
Examples	inclination( make_point( 5, 10, 0 ), make_point( 5, 10, 5 ) ) → 0.0 inclination( make_point( 5, 10, 0 ), make_point( 5, 10, 0 ) ) → 90.0 inclination( make_point( 5, 10, 0 ), make_point( 50, 100, 0 ) ) → 90.0 inclination( make_point( 5, 10, 0 ), make_point( 5, 10, -5 ) ) → 180.0

13.2.13.48. interior_ring_n

Returns a specific interior ring from a polygon geometry, or NULL if the geometry is not a polygon.

语法	interior_ring_n(geometry, index)
参数	geometry - polygon geometry index - index of interior to return, where 1 is the first interior ring
Examples	geom_to_wkt(interior_ring_n(geom_from_wkt('POLYGON((-1 -1, 4 0, 4 2, 0 2, -1 -1),(-0.1 -0.1, 0.4 0, 0.4 0.2, 0 0.2, -0.1 -0.1),(-1 -1, 4 0, 4 2, 0 2, -1 -1))'),1)) → 'LineString (-0.1 -0.1, 0.4 0, 0.4 0.2, 0 0.2, -0.1 -0.1))'

13.2.13.49. intersection

Returns a geometry that represents the shared portion of two geometries.

语法	intersection(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	geom_to_wkt( intersection( geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ), geom_from_wkt( 'LINESTRING(3 3, 4 4)' ) ) ) → 'LINESTRING(3 3, 4 4)' geom_to_wkt( intersection( geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ), geom_from_wkt( 'MULTIPOINT(3.5 3.5, 4 5)' ) ) ) → 'POINT(3.5 3.5)'

Further reading: Intersection algorithm

13.2.13.50. intersects

Tests whether a geometry intersects another. Returns TRUE if the geometries spatially intersect (share any portion of space) and false if they do not.

语法	intersects(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	intersects( geom_from_wkt( 'POINT(4 4)' ), geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ) ) → TRUE intersects( geom_from_wkt( 'POINT(4 5)' ), geom_from_wkt( 'POINT(5 5)' ) ) → FALSE

Further reading: overlay_intersects

13.2.13.51. intersects_bbox

Tests whether a geometry's bounding box overlaps another geometry's bounding box. Returns TRUE if the geometries spatially intersect the bounding box defined and false if they do not.

语法	intersects_bbox(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	intersects_bbox( geom_from_wkt( 'POINT(4 5)' ), geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ) ) → TRUE intersects_bbox( geom_from_wkt( 'POINT(6 5)' ), geom_from_wkt( 'POLYGON((3 3, 4 4, 5 5, 3 3))' ) ) → FALSE

13.2.13.52. is_closed

Returns TRUE if a line string is closed (start and end points are coincident), or false if a line string is not closed. If the geometry is not a line string then the result will be NULL.

语法	is_closed(geometry)
参数	geometry - a line string geometry
Examples	is_closed(geom_from_wkt('LINESTRING(0 0, 1 1, 2 2)')) → FALSE is_closed(geom_from_wkt('LINESTRING(0 0, 1 1, 2 2, 0 0)')) → TRUE

13.2.13.53. is_empty

Returns TRUE if a geometry is empty (without coordinates), false if the geometry is not empty and NULL if there is no geometry. See also is_empty_or_null.

语法	is_empty(geometry)
参数	geometry - a geometry
Examples	is_empty(geom_from_wkt('LINESTRING(0 0, 1 1, 2 2)')) → FALSE is_empty(geom_from_wkt('LINESTRING EMPTY')) → TRUE is_empty(geom_from_wkt('POINT(7 4)')) → FALSE is_empty(geom_from_wkt('POINT EMPTY')) → TRUE

Further reading: is_empty_or_null

13.2.13.54. is_empty_or_null

Returns TRUE if a geometry is NULL or empty (without coordinates) or false otherwise. This function is like the expression '$geometry IS NULL or is_empty($geometry)'

语法	is_empty_or_null(geometry)
参数	geometry - a geometry
Examples	is_empty_or_null(NULL) → TRUE is_empty_or_null(geom_from_wkt('LINESTRING(0 0, 1 1, 2 2)')) → FALSE is_empty_or_null(geom_from_wkt('LINESTRING EMPTY')) → TRUE is_empty_or_null(geom_from_wkt('POINT(7 4)')) → FALSE is_empty_or_null(geom_from_wkt('POINT EMPTY')) → TRUE

Further reading: is_empty, NULL

13.2.13.55. is_multipart

Returns TRUE if the geometry is of Multi type.

语法	is_multipart(geometry)
参数	geometry - a geometry
Examples	is_multipart(geom_from_wkt('MULTIPOINT ((0 0),(1 1),(2 2))')) → TRUE is_multipart(geom_from_wkt('POINT (0 0)')) → FALSE

13.2.13.56. is_valid

Returns TRUE if a geometry is valid; if it is well-formed in 2D according to the OGC rules.

语法	is_valid(geometry)
参数	geometry - a geometry
Examples	is_valid(geom_from_wkt('LINESTRING(0 0, 1 1, 2 2, 0 0)')) → TRUE is_valid(geom_from_wkt('LINESTRING(0 0)')) → FALSE

Further reading: make_valid, Check validity algorithm

13.2.13.57. $length

Returns the length of a linestring. If you need the length of a border of a polygon, use $perimeter instead. The length calculated by this function respects both the current project's ellipsoid setting and distance unit settings. For example, if an ellipsoid has been set for the project then the calculated length will be ellipsoidal, and if no ellipsoid is set then the calculated length will be planimetric.

语法	$length
Examples	$length → 42.4711

13.2.13.58. length

Returns the number of characters in a string or the length of a geometry linestring.

String variant

Returns the number of characters in a string.

语法	length(string)
参数	string - string to count length of
Examples	length('hello') → 5

Geometry variant

Calculate the length of a geometry line object. Calculations are always planimetric in the Spatial Reference System (SRS) of this geometry, and the units of the returned length will match the units for the SRS. This differs from the calculations performed by the $length function, which will perform ellipsoidal calculations based on the project's ellipsoid and distance unit settings.

语法	length(geometry)
参数	geometry - line geometry object
Examples	length(geom_from_wkt('LINESTRING(0 0, 4 0)')) → 4.0

Further reading: straight_distance_2d

13.2.13.59. length3D

Calculates the 3D length of a geometry line object. If the geometry is not a 3D line object, it returns its 2D length. Calculations are always planimetric in the Spatial Reference System (SRS) of this geometry, and the units of the returned length will match the units for the SRS. This differs from the calculations performed by the $length function, which will perform ellipsoidal calculations based on the project's ellipsoid and distance unit settings.

语法	length3D(geometry)
参数	geometry - line geometry object
Examples	length3D(geom_from_wkt('LINESTRINGZ(0 0 0, 3 0 4)')) → 5.0

13.2.13.60. line_interpolate_angle

Returns the angle parallel to the geometry at a specified distance along a linestring geometry. Angles are in degrees clockwise from north.

语法	line_interpolate_angle(geometry, distance)
参数	geometry - a linestring geometry distance - distance along line to interpolate angle at
Examples	line_interpolate_angle(geometry:=geom_from_wkt('LineString(0 0, 10 0)'),distance:=5) → 90.0

13.2.13.61. line_interpolate_point

Returns the point interpolated by a specified distance along a linestring geometry.

语法	line_interpolate_point(geometry, distance)
参数	geometry - a linestring geometry distance - distance along line to interpolate
Examples	geom_to_wkt(line_interpolate_point(geometry:=geom_from_wkt('LineString(0 0, 8 0)'), distance:=5)) → 'Point (5 0)' geom_to_wkt(line_interpolate_point(geometry:=geom_from_wkt('LineString(0 0, 1 1, 2 0)'), distance:=2.1)) → 'Point (1.48492424 0.51507576)' geom_to_wkt(line_interpolate_point(geometry:=geom_from_wkt('LineString(0 0, 1 0)'), distance:=2)) → NULL

Further reading: Interpolate point on line algorithm

13.2.13.62. line_locate_point

Returns the distance along a linestring corresponding to the closest position the linestring comes to a specified point geometry.

语法	line_locate_point(geometry, point)
参数	geometry - a linestring geometry point - point geometry to locate closest position on linestring to
Examples	line_locate_point(geometry:=geom_from_wkt('LineString(0 0, 10 0)'),point:=geom_from_wkt('Point(5 0)')) → 5.0

13.2.13.63. line_merge

Returns a LineString or MultiLineString geometry, where any connected LineStrings from the input geometry have been merged into a single linestring. This function will return NULL if passed a geometry which is not a LineString/MultiLineString.

语法	line_merge(geometry)
参数	geometry - a LineString/MultiLineString geometry
Examples	geom_to_wkt(line_merge(geom_from_wkt('MULTILINESTRING((0 0, 1 1),(1 1, 2 2))'))) → 'LineString(0 0,1 1,2 2)' geom_to_wkt(line_merge(geom_from_wkt('MULTILINESTRING((0 0, 1 1),(11 1, 21 2))'))) → 'MultiLineString((0 0, 1 1),(11 1, 21 2)'

13.2.13.64. line_substring

Returns the portion of a line (or curve) geometry which falls between the specified start and end distances (measured from the beginning of the line). Z and M values are linearly interpolated from existing values.

语法	line_substring(geometry, start_distance, end_distance)
参数	geometry - a linestring or curve geometry start_distance - distance to start of substring end_distance - distance to end of substring
Examples	geom_to_wkt(line_substring(geometry:=geom_from_wkt('LineString(0 0, 10 0)'),start_distance:=2,end_distance:=6)) → 'LineString (2 0,6 0)'

Further reading: Line substring algorithm

13.2.13.65. m

Returns the m (measure) value of a point geometry.

语法	m(geometry)
参数	geometry - a point geometry
Examples	m( geom_from_wkt( 'POINTM(2 5 4)' ) ) → 4

13.2.13.66. m_max

Returns the maximum m (measure) value of a geometry.

语法	m_max(geometry)
参数	geometry - a geometry containing m values
Examples	m_max( make_point_m( 0,0,1 ) ) → 1 m_max(make_line( make_point_m( 0,0,1 ), make_point_m( -1,-1,2 ), make_point_m( -2,-2,0 ) ) ) → 2

13.2.13.67. m_min

Returns the minimum m (measure) value of a geometry.

语法	m_min(geometry)
参数	geometry - a geometry containing m values
Examples	m_min( make_point_m( 0,0,1 ) ) → 1 m_min(make_line( make_point_m( 0,0,1 ), make_point_m( -1,-1,2 ), make_point_m( -2,-2,0 ) ) ) → 0

13.2.13.68. main_angle

Returns the angle of the long axis (clockwise, in degrees from North) of the oriented minimal bounding rectangle, which completely covers the geometry.

语法	main_angle(geometry)
参数	geometry - a geometry
Examples	main_angle(geom_from_wkt('Polygon ((321577 129614, 321581 129618, 321585 129615, 321581 129610, 321577 129614))')) → 38.66

13.2.13.69. make_circle

Creates a circular polygon.

语法	make_circle(center, radius, [segments=36]) [] 标记可选参数
参数	center - center point of the circle radius - radius of the circle segments - optional argument for polygon segmentation. By default this value is 36
Examples	geom_to_wkt(make_circle(make_point(10,10), 5, 4)) → 'Polygon ((10 15, 15 10, 10 5, 5 10, 10 15))' geom_to_wkt(make_circle(make_point(10,10,5), 5, 4)) → 'PolygonZ ((10 15 5, 15 10 5, 10 5 5, 5 10 5, 10 15 5))' geom_to_wkt(make_circle(make_point(10,10,5,30), 5, 4)) → 'PolygonZM ((10 15 5 30, 15 10 5 30, 10 5 5 30, 5 10 5 30, 10 15 5 30))'

13.2.13.70. make_ellipse

Creates an elliptical polygon.

语法	make_ellipse(center, semi_major_axis, semi_minor_axis, azimuth, [segments=36]) [] 标记可选参数
参数	center - center point of the ellipse semi_major_axis - semi-major axis of the ellipse semi_minor_axis - semi-minor axis of the ellipse azimuth - orientation of the ellipse segments - optional argument for polygon segmentation. By default this value is 36
Examples	geom_to_wkt(make_ellipse(make_point(10,10), 5, 2, 90, 4)) → 'Polygon ((15 10, 10 8, 5 10, 10 12, 15 10))' geom_to_wkt(make_ellipse(make_point(10,10,5), 5, 2, 90, 4)) → 'PolygonZ ((15 10 5, 10 8 5, 5 10 5, 10 12 5, 15 10 5))' geom_to_wkt(make_ellipse(make_point(10,10,5,30), 5, 2, 90, 4)) → 'PolygonZM ((15 10 5 30, 10 8 5 30, 5 10 5 30, 10 12 5 30, 15 10 5 30))'

13.2.13.71. make_line

Creates a line geometry from a series of point geometries.

List of arguments variant

Line vertices are specified as separate arguments to the function.

语法	make_line(point1, point2, ...)
参数	point - a point geometry (or array of points)
Examples	geom_to_wkt(make_line(make_point(2,4),make_point(3,5))) → 'LineString (2 4, 3 5)' geom_to_wkt(make_line(make_point(2,4),make_point(3,5),make_point(9,7))) → 'LineString (2 4, 3 5, 9 7)'

Array variant

Line vertices are specified as an array of points.

语法	make_line(array)
参数	array - array of points
Examples	geom_to_wkt(make_line(array(make_point(2,4),make_point(3,5),make_point(9,7)))) → 'LineString (2 4, 3 5, 9 7)'

13.2.13.72. make_point

Creates a point geometry from an x and y (and optional z and m) value.

语法	make_point(x, y, [z], [m]) [] 标记可选参数
参数	x - x coordinate of point y - y coordinate of point z - optional z coordinate of point m - optional m value of point
Examples	geom_to_wkt(make_point(2,4)) → 'Point (2 4)' geom_to_wkt(make_point(2,4,6)) → 'PointZ (2 4 6)' geom_to_wkt(make_point(2,4,6,8)) → 'PointZM (2 4 6 8)'

13.2.13.73. make_point_m

Creates a point geometry from an x, y coordinate and m value.

语法	make_point_m(x, y, m)
参数	x - x coordinate of point y - y coordinate of point m - m value of point
Examples	geom_to_wkt(make_point_m(2,4,6)) → 'PointM (2 4 6)'

13.2.13.74. make_polygon

Creates a polygon geometry from an outer ring and optional series of inner ring geometries.

语法	make_polygon(outerRing, [innerRing1], [innerRing2], ...) [] 标记可选参数
参数	outerRing - closed line geometry for polygon's outer ring innerRing - optional closed line geometry for inner ring
Examples	geom_to_wkt(make_polygon(geom_from_wkt('LINESTRING( 0 0, 0 1, 1 1, 1 0, 0 0 )'))) → 'Polygon ((0 0, 0 1, 1 1, 1 0, 0 0))' geom_to_wkt(make_polygon(geom_from_wkt('LINESTRING( 0 0, 0 1, 1 1, 1 0, 0 0 )'),geom_from_wkt('LINESTRING( 0.1 0.1, 0.1 0.2, 0.2 0.2, 0.2 0.1, 0.1 0.1 )'),geom_from_wkt('LINESTRING( 0.8 0.8, 0.8 0.9, 0.9 0.9, 0.9 0.8, 0.8 0.8 )'))) → 'Polygon ((0 0, 0 1, 1 1, 1 0, 0 0),(0.1 0.1, 0.1 0.2, 0.2 0.2, 0.2 0.1, 0.1 0.1),(0.8 0.8, 0.8 0.9, 0.9 0.9, 0.9 0.8, 0.8 0.8))'

13.2.13.75. make_rectangle_3points

Creates a rectangle from 3 points.

语法	make_rectangle_3points(point1, point2, point3, [option=0]) [] 标记可选参数
参数	point1 - First point. point2 - Second point. point3 - Third point. option - An optional argument to construct the rectangle. By default this value is 0. Value can be 0 (distance) or 1 (projected). Option distance: Second distance is equal to the distance between 2nd and 3rd point. Option projected: Second distance is equal to the distance of the perpendicular projection of the 3rd point on the segment or its extension.
Examples	geom_to_wkt(make_rectangle_3points(make_point(0, 0), make_point(0,5), make_point(5, 5), 0)) → 'Polygon ((0 0, 0 5, 5 5, 5 0, 0 0))' geom_to_wkt(make_rectangle_3points(make_point(0, 0), make_point(0,5), make_point(5, 3), 1)) → 'Polygon ((0 0, 0 5, 5 5, 5 0, 0 0))'

13.2.13.76. make_regular_polygon

Creates a regular polygon.

语法	make_regular_polygon(center, radius, number_sides, [circle=0]) [] 标记可选参数
参数	center - center of the regular polygon radius - second point. The first if the regular polygon is inscribed. The midpoint of the first side if the regular polygon is circumscribed. number_sides - Number of sides/edges of the regular polygon circle - Optional argument to construct the regular polygon. By default this value is 0. Value can be 0 (inscribed) or 1 (circumscribed)
Examples	geom_to_wkt(make_regular_polygon(make_point(0,0), make_point(0,5), 5)) → 'Polygon ((0 5, 4.76 1.55, 2.94 -4.05, -2.94 -4.05, -4.76 1.55, 0 5))' geom_to_wkt(make_regular_polygon(make_point(0,0), project(make_point(0,0), 4.0451, radians(36)), 5)) → 'Polygon ((0 5, 4.76 1.55, 2.94 -4.05, -2.94 -4.05, -4.76 1.55, 0 5))'

13.2.13.77. make_square

Creates a square from a diagonal.

语法	make_square(point1, point2)
参数	point1 - First point of the diagonal point2 - Last point of the diagonal
Examples	geom_to_wkt(make_square( make_point(0,0), make_point(5,5))) → 'Polygon ((0 0, -0 5, 5 5, 5 0, 0 0))' geom_to_wkt(make_square( make_point(5,0), make_point(5,5))) → 'Polygon ((5 0, 2.5 2.5, 5 5, 7.5 2.5, 5 0))'

13.2.13.78. make_triangle

Creates a triangle polygon.

语法	make_triangle(point1, point2, point3)
参数	point1 - first point of the triangle point2 - second point of the triangle point3 - third point of the triangle
Examples	geom_to_wkt(make_triangle(make_point(0,0), make_point(5,5), make_point(0,10))) → 'Triangle ((0 0, 5 5, 0 10, 0 0))' geom_to_wkt(boundary(make_triangle(make_point(0,0), make_point(5,5), make_point(0,10)))) → 'LineString (0 0, 5 5, 0 10, 0 0)'

13.2.13.79. make_valid

Returns a valid geometry or an empty geometry if the geometry could not be made valid.

语法	make_valid(geometry, [method=structure], [keep_collapsed=false]) [] 标记可选参数
参数	geometry - a geometry method - repair algorithm. May be either 'structure' or 'linework'. The 'linework' option combines all rings into a set of noded lines and then extracts valid polygons from that linework. The 'structure' method first makes all rings valid and then merges shells and subtracts holes from shells to generate valid result. Assumes that holes and shells are correctly categorized. keep_collapsed - if set to true, then components that have collapsed into a lower dimensionality will be kept. For example, a ring collapsing to a line, or a line collapsing to a point.
Examples	geom_to_wkt(make_valid(geom_from_wkt('POLYGON((3 2, 4 1, 5 8, 3 2, 4 2))'))) → 'Polygon ((3 2, 5 8, 4 1, 3 2))' geom_to_wkt(make_valid(geom_from_wkt('POLYGON((3 2, 4 1, 5 8, 3 2, 4 2))'), 'linework')) → 'GeometryCollection (Polygon ((5 8, 4 1, 3 2, 5 8)),LineString (3 2, 4 2))' geom_to_wkt(make_valid(geom_from_wkt('POLYGON((3 2, 4 1, 5 8))'), method:='linework')) → 'Polygon ((3 2, 4 1, 5 8, 3 2))' make_valid(geom_from_wkt('LINESTRING(0 0)')) → An empty geometry

Further reading: is_valid, Fix geometries algorithm

13.2.13.80. minimal_circle

Returns the minimal enclosing circle of a geometry. It represents the minimum circle that encloses all geometries within the set.

语法	minimal_circle(geometry, [segments=36]) [] 标记可选参数
参数	geometry - a geometry segments - optional argument for polygon segmentation. By default this value is 36
Examples	geom_to_wkt( minimal_circle( geom_from_wkt( 'LINESTRING(0 5, 0 -5, 2 1)' ), 4 ) ) → 'Polygon ((0 5, 5 -0, -0 -5, -5 0, 0 5))' geom_to_wkt( minimal_circle( geom_from_wkt( 'MULTIPOINT(1 2, 3 4, 3 2)' ), 4 ) ) → 'Polygon ((3 4, 3 2, 1 2, 1 4, 3 4))'

Further reading: Minimum enclosing circles algorithm

13.2.13.81. nodes_to_points

Returns a multipoint geometry consisting of every node in the input geometry.

语法	nodes_to_points(geometry, [ignore_closing_nodes=false]) [] 标记可选参数
参数	geometry - geometry object ignore_closing_nodes - optional argument specifying whether to include duplicate nodes which close lines or polygons rings. Defaults to false, set to true to avoid including these duplicate nodes in the output collection.
Examples	geom_to_wkt(nodes_to_points(geom_from_wkt('LINESTRING(0 0, 1 1, 2 2)'))) → 'MultiPoint ((0 0),(1 1),(2 2))' geom_to_wkt(nodes_to_points(geom_from_wkt('POLYGON((-1 -1, 4 0, 4 2, 0 2, -1 -1))'),true)) → 'MultiPoint ((-1 -1),(4 0),(4 2),(0 2))'

Further reading: Extract vertices algorithm

13.2.13.82. num_geometries

Returns the number of geometries in a geometry collection, or the number of parts in a multi-part geometry. The function returns NULL if the input geometry is not a collection.

语法	num_geometries(geometry)
参数	geometry - geometry collection or multi-part geometry
Examples	num_geometries(geom_from_wkt('GEOMETRYCOLLECTION(POINT(0 1), POINT(0 0), POINT(1 0), POINT(1 1))')) → 4 num_geometries(geom_from_wkt('MULTIPOINT((0 1), (0 0), (1 0))')) → 3

13.2.13.83. num_interior_rings

Returns the number of interior rings in a polygon or geometry collection, or NULL if the input geometry is not a polygon or collection.

语法	num_interior_rings(geometry)
参数	geometry - input geometry
Examples	num_interior_rings(geom_from_wkt('POLYGON((-1 -1, 4 0, 4 2, 0 2, -1 -1),(-0.1 -0.1, 0.4 0, 0.4 0.2, 0 0.2, -0.1 -0.1))')) → 1

13.2.13.84. num_points

Returns the number of vertices in a geometry.

语法	num_points(geometry)
参数	geometry - a geometry
Examples	num_points($geometry) → number of vertices in the current feature's geometry

13.2.13.85. num_rings

Returns the number of rings (including exterior rings) in a polygon or geometry collection, or NULL if the input geometry is not a polygon or collection.

语法	num_rings(geometry)
参数	geometry - input geometry
Examples	num_rings(geom_from_wkt('POLYGON((-1 -1, 4 0, 4 2, 0 2, -1 -1),(-0.1 -0.1, 0.4 0, 0.4 0.2, 0 0.2, -0.1 -0.1))')) → 2

13.2.13.86. offset_curve

Returns a geometry formed by offsetting a linestring geometry to the side. Distances are in the Spatial Reference System of this geometry.

语法	offset_curve(geometry, distance, [segments=8], [join=1], [miter_limit=2.0]) [] 标记可选参数
参数	geometry - a (multi)linestring geometry distance - offset distance. Positive values will be buffered to the left of lines, negative values to the right segments - number of segments to use to represent a quarter circle when a round join style is used. A larger number results in a smoother line with more nodes. join - join style for corners, where 1 = round, 2 = miter and 3 = bevel miter_limit - limit on the miter ratio used for very sharp corners (when using miter joins only)
Examples	offset_curve($geometry, 10.5) → line offset to the left by 10.5 units offset_curve($geometry, -10.5) → line offset to the right by 10.5 units offset_curve($geometry, 10.5, segments:=16, join:=1) → line offset to the left by 10.5 units, using more segments to result in a smoother curve offset_curve($geometry, 10.5, join:=3) → line offset to the left by 10.5 units, using a beveled join

Further reading: Offset lines algorithm

13.2.13.87. order_parts

Orders the parts of a MultiGeometry by a given criteria

语法	order_parts(geometry, orderby, [ascending=true]) [] 标记可选参数
参数	geometry - a multi-type geometry orderby - an expression string defining the order criteria ascending - boolean, True for ascending, False for descending
Examples	geom_to_wkt(order_parts(geom_from_wkt('MultiPolygon (((1 1, 5 1, 5 5, 1 5, 1 1)),((1 1, 9 1, 9 9, 1 9, 1 1)))'), 'area($geometry)', False)) → 'MultiPolygon (((1 1, 9 1, 9 9, 1 9, 1 1)),((1 1, 5 1, 5 5, 1 5, 1 1)))' geom_to_wkt(order_parts(geom_from_wkt('LineString(1 2, 3 2, 4 3)'), '1', True)) → 'LineString(1 2, 3 2, 4 3)'

13.2.13.88. oriented_bbox

Returns a geometry which represents the minimal oriented bounding box of an input geometry.

语法	oriented_bbox(geometry)
参数	geometry - a geometry
Examples	geom_to_wkt( oriented_bbox( geom_from_wkt( 'MULTIPOINT(1 2, 3 4, 3 2)' ) ) ) → 'Polygon ((3 2, 3 4, 1 4, 1 2, 3 2))'

Further reading: Oriented minimum bounding box algorithm

13.2.13.89. overlaps

Tests whether a geometry overlaps another. Returns TRUE if the geometries share space, are of the same dimension, but are not completely contained by each other.

语法	overlaps(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	overlaps( geom_from_wkt( 'LINESTRING(3 5, 4 4, 5 5, 5 3)' ), geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ) ) → TRUE overlaps( geom_from_wkt( 'LINESTRING(0 0, 1 1)' ), geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ) ) → FALSE

13.2.13.90. overlay_contains

Returns whether the current feature spatially contains at least one feature from a target layer, or an array of expression-based results for the features in the target layer contained in the current feature.

Read more on the underlying GEOS "Contains" predicate, as described in PostGIS ST_Contains function.

语法	overlay_contains(layer, [expression], [filter], [limit], [cache=false]) [] 标记可选参数
参数	layer - the layer whose overlay is checked expression - an optional expression to evaluate on the features from the target layer. If not set, the function will just return a boolean indicating whether there is at least one match. filter - an optional expression to filter the target features to check. If not set, all the features will be checked. limit - an optional integer to limit the number of matching features. If not set, all the matching features will be returned. cache - set this to true to build a local spatial index (most of the time, this is unwanted, unless you are working with a particularly slow data provider)
Examples	overlay_contains('regions') → TRUE if the current feature spatially contains a region overlay_contains('regions', filter:= population > 10000) → TRUE if the current feature spatially contains a region with a population greater than 10000 overlay_contains('regions', name) → an array of names, for the regions contained in the current feature array_to_string(overlay_contains('regions', name)) → a string as a comma separated list of names, for the regions contained in the current feature array_sort(overlay_contains(layer:='regions', expression:="name", filter:= population > 10000)) → an ordered array of names, for the regions contained in the current feature and with a population greater than 10000 overlay_contains(layer:='regions', expression:= geom_to_wkt($geometry), limit:=2) → an array of geometries (in WKT), for up to two regions contained in the current feature

Further reading: contains, array manipulation, Select by location algorithm

13.2.13.91. overlay_crosses

Returns whether the current feature spatially crosses at least one feature from a target layer, or an array of expression-based results for the features in the target layer crossed by the current feature.

Read more on the underlying GEOS "Crosses" predicate, as described in PostGIS ST_Crosses function.

语法	overlay_crosses(layer, [expression], [filter], [limit], [cache=false]) [] 标记可选参数
参数	layer - the layer whose overlay is checked expression - an optional expression to evaluate on the features from the target layer. If not set, the function will just return a boolean indicating whether there is at least one match. filter - an optional expression to filter the target features to check. If not set, all the features will be checked. limit - an optional integer to limit the number of matching features. If not set, all the matching features will be returned. cache - set this to true to build a local spatial index (most of the time, this is unwanted, unless you are working with a particularly slow data provider)
Examples	overlay_crosses('regions') → TRUE if the current feature spatially crosses a region overlay_crosses('regions', filter:= population > 10000) → TRUE if the current feature spatially crosses a region with a population greater than 10000 overlay_crosses('regions', name) → an array of names, for the regions crossed by the current feature array_to_string(overlay_crosses('regions', name)) → a string as a comma separated list of names, for the regions crossed by the current feature array_sort(overlay_crosses(layer:='regions', expression:="name", filter:= population > 10000)) → an ordered array of names, for the regions crossed by the current feature and with a population greater than 10000 overlay_crosses(layer:='regions', expression:= geom_to_wkt($geometry), limit:=2) → an array of geometries (in WKT), for up to two regions crossed by the current feature

Further reading: crosses, array manipulation, Select by location algorithm

13.2.13.92. overlay_disjoint

Returns whether the current feature is spatially disjoint from all the features of a target layer, or an array of expression-based results for the features in the target layer that are disjoint from the current feature.

Read more on the underlying GEOS "Disjoint" predicate, as described in PostGIS ST_Disjoint function.

语法	overlay_disjoint(layer, [expression], [filter], [limit], [cache=false]) [] 标记可选参数
参数	layer - the layer whose overlay is checked expression - an optional expression to evaluate on the features from the target layer. If not set, the function will just return a boolean indicating whether there is at least one match. filter - an optional expression to filter the target features to check. If not set, all the features will be checked. limit - an optional integer to limit the number of matching features. If not set, all the matching features will be returned. cache - set this to true to build a local spatial index (most of the time, this is unwanted, unless you are working with a particularly slow data provider)
Examples	overlay_disjoint('regions') → TRUE if the current feature is spatially disjoint from all the regions overlay_disjoint('regions', filter:= population > 10000) → TRUE if the current feature is spatially disjoint from all the regions with a population greater than 10000 overlay_disjoint('regions', name) → an array of names, for the regions spatially disjoint from the current feature array_to_string(overlay_disjoint('regions', name)) → a string as a comma separated list of names, for the regions spatially disjoint from the current feature array_sort(overlay_disjoint(layer:='regions', expression:="name", filter:= population > 10000)) → an ordered array of names, for the regions spatially disjoint from the current feature and with a population greater than 10000 overlay_disjoint(layer:='regions', expression:= geom_to_wkt($geometry), limit:=2) → an array of geometries (in WKT), for up to two regions spatially disjoint from the current feature

Further reading: disjoint, array manipulation, Select by location algorithm

13.2.13.93. overlay_equals

Returns whether the current feature spatially equals to at least one feature from a target layer, or an array of expression-based results for the features in the target layer that are spatially equal to the current feature.

Read more on the underlying GEOS "Equals" predicate, as described in PostGIS ST_Equals function.

语法	overlay_equals(layer, [expression], [filter], [limit], [cache=false]) [] 标记可选参数
参数	layer - the layer whose overlay is checked expression - an optional expression to evaluate on the features from the target layer. If not set, the function will just return a boolean indicating whether there is at least one match. filter - an optional expression to filter the target features to check. If not set, all the features will be checked. limit - an optional integer to limit the number of matching features. If not set, all the matching features will be returned. cache - set this to true to build a local spatial index (most of the time, this is unwanted, unless you are working with a particularly slow data provider)
Examples	overlay_equals('regions') → TRUE if the current feature is spatially equal to a region overlay_equals('regions', filter:= population > 10000) → TRUE if the current feature is spatially equal to a region with a population greater than 10000 overlay_equals('regions', name) → an array of names, for the regions spatially equal to the current feature array_to_string(overlay_equals('regions', name)) → a string as a comma separated list of names, for the regions spatially equal to the current feature array_sort(overlay_equals(layer:='regions', expression:="name", filter:= population > 10000)) → an ordered array of names, for the regions spatially equal to the current feature and with a population greater than 10000 overlay_equals(layer:='regions', expression:= geom_to_wkt($geometry), limit:=2) → an array of geometries (in WKT), for up to two regions spatially equal to the current feature

Further reading: array manipulation, Select by location algorithm

13.2.13.94. overlay_intersects

Returns whether the current feature spatially intersects at least one feature from a target layer, or an array of expression-based results for the features in the target layer intersected by the current feature.

Read more on the underlying GEOS "Intersects" predicate, as described in PostGIS ST_Intersects function.

语法	overlay_intersects(layer, [expression], [filter], [limit], [cache=false], [min_overlap], [min_inscribed_circle_radius], [return_details], [sort_by_intersection_size]) [] 标记可选参数
参数	layer - the layer whose overlay is checked expression - an optional expression to evaluate on the features from the target layer. If not set, the function will just return a boolean indicating whether there is at least one match. filter - an optional expression to filter the target features to check. If not set, all the features will be checked. limit - an optional integer to limit the number of matching features. If not set, all the matching features will be returned. cache - set this to true to build a local spatial index (most of the time, this is unwanted, unless you are working with a particularly slow data provider) min_overlap - defines an optional exclusion filter: for polygons, a minimum area in current feature squared units for the intersection. If the intersection results in multiple polygons the intersection will be returned if at least one of the polygons has an area greater or equal to the value for lines, a minimum length in current feature units. If the intersection results in multiple lines the intersection will be returned if at least one of the lines has a length greater or equal to the value. min_inscribed_circle_radius - defines an optional exclusion filter (for polygons only): minimum radius in current feature units for the maximum inscribed circle of the intersection. If the intersection results in multiple polygons the intersection will be returned if at least one of the polygons has a radius for the maximum inscribed circle greater or equal to the value. Read more on the underlying GEOS predicate, as described in PostGIS ST_MaximumInscribedCircle function. This argument requires GEOS >= 3.9. return_details - Set this to true to return a list of maps containing (key names in quotes) the feature 'id', the expression 'result' and the 'overlap' value. The 'radius' of the maximum inscribed circle is also returned when the target layer is a polygon. Only valid when used with the expression parameter sort_by_intersection_size - only valid when used with an expression, set this to 'des' to return the results ordered by the overlap value in descending order or set this to 'asc' for ascending order.
Examples	overlay_intersects('regions') → TRUE if the current feature spatially intersects a region overlay_intersects('regions', filter:= population > 10000) → TRUE if the current feature spatially intersects a region with a population greater than 10000 overlay_intersects('regions', name) → an array of names, for the regions intersected by the current feature array_to_string(overlay_intersects('regions', name)) → a string as a comma separated list of names, for the regions intersected by the current feature array_sort(overlay_intersects(layer:='regions', expression:="name", filter:= population > 10000)) → an ordered array of names, for the regions intersected by the current feature and with a population greater than 10000 overlay_intersects(layer:='regions', expression:= geom_to_wkt($geometry), limit:=2) → an array of geometries (in WKT), for up to two regions intersected by the current feature overlay_intersects(layer:='regions', min_overlap:=0.54) → TRUE if the current feature spatially intersects a region and the intersection area (of at least one of the parts in case of multipolygons) is greater or equal to 0.54 overlay_intersects(layer:='regions', min_inscribed_circle_radius:=0.54) → TRUE if the current feature spatially intersects a region and the intersection area maximum inscribed circle's radius (of at least one of the parts in case of multipart) is greater or equal to 0.54 overlay_intersects(layer:='regions', expression:= geom_to_wkt($geometry), return_details:=true) → an array of maps containing 'id', 'result', 'overlap' and 'radius' overlay_intersects(layer:='regions', expression:= geom_to_wkt($geometry), sort_by_intersection_size:='des') → an array of geometries (in WKT) ordered by the overlap value in descending order

Further reading: intersects, array manipulation, Select by location algorithm

13.2.13.95. overlay_nearest

Returns whether the current feature has feature(s) from a target layer within a given distance, or an array of expression-based results for the features in the target layer within a distance from the current feature.

Note: This function can be slow and consume a lot of memory for large layers.

语法	overlay_nearest(layer, [expression], [filter], [limit=1], [max_distance], [cache=false]) [] 标记可选参数
参数	layer - the target layer expression - an optional expression to evaluate on the features from the target layer. If not set, the function will just return a boolean indicating whether there is at least one match. filter - an optional expression to filter the target features to check. If not set, all the features in the target layer will be used. limit - an optional integer to limit the number of matching features. If not set, only the nearest feature will be returned. If set to -1, returns all the matching features. max_distance - an optional distance to limit the search of matching features. If not set, all the features in the target layer will be used. cache - set this to true to build a local spatial index (most of the time, this is unwanted, unless you are working with a particularly slow data provider)
Examples	overlay_nearest('airports') → TRUE if the "airports" layer has at least one feature overlay_nearest('airports', max_distance:= 5000) → TRUE if there is an airport within a distance of 5000 map units from the current feature overlay_nearest('airports', name) → the name of the closest airport to the current feature, as an array array_to_string(overlay_nearest('airports', name)) → the name of the closest airport to the current feature, as a string overlay_nearest(layer:='airports', expression:= name, max_distance:= 5000) → the name of the closest airport within a distance of 5000 map units from the current feature, as an array overlay_nearest(layer:='airports', expression:="name", filter:= "Use"='Civilian', limit:=3) → an array of names, for up to the three closest civilian airports ordered by distance overlay_nearest(layer:='airports', expression:="name", limit:= -1, max_distance:= 5000) → an array of names, for all the airports within a distance of 5000 map units from the current feature, ordered by distance

Further reading: array manipulation, Join attributes by nearest algorithm

13.2.13.96. overlay_touches

Returns whether the current feature spatially touches at least one feature from a target layer, or an array of expression-based results for the features in the target layer touched by the current feature.

Read more on the underlying GEOS "Touches" predicate, as described in PostGIS ST_Touches function.

语法	overlay_touches(layer, [expression], [filter], [limit], [cache=false]) [] 标记可选参数
参数	layer - the layer whose overlay is checked expression - an optional expression to evaluate on the features from the target layer. If not set, the function will just return a boolean indicating whether there is at least one match. filter - an optional expression to filter the target features to check. If not set, all the features will be checked. limit - an optional integer to limit the number of matching features. If not set, all the matching features will be returned. cache - set this to true to build a local spatial index (most of the time, this is unwanted, unless you are working with a particularly slow data provider)
Examples	overlay_touches('regions') → TRUE if the current feature spatially touches a region overlay_touches('regions', filter:= population > 10000) → TRUE if the current feature spatially touches a region with a population greater than 10000 overlay_touches('regions', name) → an array of names, for the regions touched by the current feature string_to_array(overlay_touches('regions', name)) → a string as a comma separated list of names, for the regions touched by the current feature array_sort(overlay_touches(layer:='regions', expression:="name", filter:= population > 10000)) → an ordered array of names, for the regions touched by the current feature and with a population greater than 10000 overlay_touches(layer:='regions', expression:= geom_to_wkt($geometry), limit:=2) → an array of geometries (in WKT), for up to two regions touched by the current feature

Further reading: touches, array manipulation, Select by location algorithm

13.2.13.97. overlay_within

Returns whether the current feature is spatially within at least one feature from a target layer, or an array of expression-based results for the features in the target layer that contain the current feature.

Read more on the underlying GEOS "Within" predicate, as described in PostGIS ST_Within function.

语法	overlay_within(layer, [expression], [filter], [limit], [cache=false]) [] 标记可选参数
参数	layer - the layer whose overlay is checked expression - an optional expression to evaluate on the features from the target layer. If not set, the function will just return a boolean indicating whether there is at least one match. filter - an optional expression to filter the target features to check. If not set, all the features will be checked. limit - an optional integer to limit the number of matching features. If not set, all the matching features will be returned. cache - set this to true to build a local spatial index (most of the time, this is unwanted, unless you are working with a particularly slow data provider)
Examples	overlay_within('regions') → TRUE if the current feature is spatially within a region overlay_within('regions', filter:= population > 10000) → TRUE if the current feature is spatially within a region with a population greater than 10000 overlay_within('regions', name) → an array of names, for the regions containing the current feature array_to_string(overlay_within('regions', name)) → a string as a comma separated list of names, for the regions containing the current feature array_sort(overlay_within(layer:='regions', expression:="name", filter:= population > 10000)) → an ordered array of names, for the regions containing the current feature and with a population greater than 10000 overlay_within(layer:='regions', expression:= geom_to_wkt($geometry), limit:=2) → an array of geometries (in WKT), for up to two regions containing the current feature

Further reading: within, array manipulation, Select by location algorithm

13.2.13.98. $perimeter

Returns the perimeter length of the current feature. The perimeter calculated by this function respects both the current project's ellipsoid setting and distance unit settings. For example, if an ellipsoid has been set for the project then the calculated perimeter will be ellipsoidal, and if no ellipsoid is set then the calculated perimeter will be planimetric.

语法	$perimeter
Examples	$perimeter → 42

13.2.13.99. perimeter

Returns the perimeter of a geometry polygon object. Calculations are always planimetric in the Spatial Reference System (SRS) of this geometry, and the units of the returned perimeter will match the units for the SRS. This differs from the calculations performed by the $perimeter function, which will perform ellipsoidal calculations based on the project's ellipsoid and distance unit settings.

语法	perimeter(geometry)
参数	geometry - polygon geometry object
Examples	perimeter(geom_from_wkt('POLYGON((0 0, 4 0, 4 2, 0 2, 0 0))')) → 12.0

13.2.13.100. point_n

Returns a specific node from a geometry.

语法	point_n(geometry, index)
参数	geometry - geometry object index - index of node to return, where 1 is the first node; if the value is negative, the selected vertex index will be its total count minus the absolute value
Examples	geom_to_wkt(point_n(geom_from_wkt('POLYGON((0 0, 4 0, 4 2, 0 2, 0 0))'),2)) → 'Point (4 0)'

Further reading: Extract specific vertices algorithm

13.2.13.101. point_on_surface

Returns a point guaranteed to lie on the surface of a geometry.

语法	point_on_surface(geometry)
参数	geometry - a geometry
Examples	point_on_surface($geometry) → a point geometry

Further reading: Point on Surface algorithm

13.2.13.102. pole_of_inaccessibility

Calculates the approximate pole of inaccessibility for a surface, which is the most distant internal point from the boundary of the surface. This function uses the 'polylabel' algorithm (Vladimir Agafonkin, 2016), which is an iterative approach guaranteed to find the true pole of inaccessibility within a specified tolerance. More precise tolerances require more iterations and will take longer to calculate.

语法	pole_of_inaccessibility(geometry, tolerance)
参数	geometry - a geometry tolerance - maximum distance between the returned point and the true pole location
Examples	geom_to_wkt(pole_of_inaccessibility( geom_from_wkt('POLYGON((0 1, 0 9, 3 10, 3 3, 10 3, 10 1, 0 1))'), 0.1)) → 'Point(1.546875 2.546875)'

Further reading: Pole of inaccessibility algorithm

13.2.13.103. project

Returns a point projected from a start point using a distance, a bearing (azimuth) and an elevation in radians.

语法	project(point, distance, azimuth, [elevation]) [] 标记可选参数
参数	point - start point distance - distance to project azimuth - azimuth in radians clockwise, where 0 corresponds to north elevation - angle of inclination in radians
Examples	geom_to_wkt(project(make_point(1, 2), 3, radians(270))) → 'Point(-2, 2)'

Further reading: Project points (Cartesian) algorithm

13.2.13.104. relate

Tests the Dimensional Extended 9 Intersection Model (DE-9IM) representation of the relationship between two geometries.

Relationship variant

Returns the Dimensional Extended 9 Intersection Model (DE-9IM) representation of the relationship between two geometries.

语法	relate(geometry, geometry)
参数	geometry - a geometry geometry - a geometry
Examples	relate( geom_from_wkt( 'LINESTRING(40 40,120 120)' ), geom_from_wkt( 'LINESTRING(40 40,60 120)' ) ) → 'FF1F00102'

Pattern match variant

Tests whether the DE-9IM relationship between two geometries matches a specified pattern.

语法	relate(geometry, geometry, pattern)
参数	geometry - a geometry geometry - a geometry pattern - DE-9IM pattern to match
Examples	relate( geom_from_wkt( 'LINESTRING(40 40,120 120)' ), geom_from_wkt( 'LINESTRING(40 40,60 120)' ), '**1F001**' ) → TRUE

13.2.13.105. reverse

Reverses the direction of a line string by reversing the order of its vertices.

语法	reverse(geometry)
参数	geometry - a geometry
Examples	geom_to_wkt(reverse(geom_from_wkt('LINESTRING(0 0, 1 1, 2 2)'))) → 'LINESTRING(2 2, 1 1, 0 0)'

Further reading: Reverse line direction algorithm

13.2.13.106. rotate

Returns a rotated version of a geometry. Calculations are in the Spatial Reference System of this geometry.

语法	rotate(geometry, rotation, [center=NULL], [per_part=false]) [] 标记可选参数
参数	geometry - a geometry rotation - clockwise rotation in degrees center - rotation center point. If not specified, the center of the geometry's bounding box is used. per_part - apply rotation per part. If true, then rotation will apply around the center of each part's bounding box when the input geometry is multipart and an explicit rotation center point is not specified.
Examples	rotate($geometry, 45, make_point(4, 5)) → geometry rotated 45 degrees clockwise around the (4, 5) point rotate($geometry, 45) → geometry rotated 45 degrees clockwise around the center of its bounding box

图 13.7 Rotating features

13.2.13.107. roundness

Calculates how close a polygon shape is to a circle. The function Returns TRUE when the polygon shape is a perfect circle and 0 when it is completely flat.

语法	roundness(geometry)
参数	geometry - a polygon
Examples	round(roundness(geom_from_wkt('POLYGON(( 0 0, 0 1, 1 1, 1 0, 0 0))')), 3) → 0.785 round(roundness(geom_from_wkt('POLYGON(( 0 0, 0 0.1, 1 0.1, 1 0, 0 0))')), 3) → 0.260

Further reading: Roundness algorithm

13.2.13.108. scale

Returns a scaled version of a geometry. Calculations are in the Spatial Reference System of this geometry.

语法	scale(geometry, x_scale, y_scale, [center]) [] 标记可选参数
参数	geometry - a geometry x_scale - x-axis scaling factor y_scale - y-axis scaling factor center - scaling center point. If not specified, the center of the geometry's bounding box is used.
Examples	scale($geometry, 2, 0.5, make_point(4, 5)) → geometry scaled twice horizontally and halved vertically, around the (4, 5) point scale($geometry, 2, 0.5) → geometry twice horizontally and halved vertically, around the center of its bounding box

13.2.13.109. segments_to_lines

Returns a multi line geometry consisting of a line for every segment in the input geometry.

语法	segments_to_lines(geometry)
参数	geometry - geometry object
Examples	geom_to_wkt(segments_to_lines(geom_from_wkt('LINESTRING(0 0, 1 1, 2 2)'))) → 'MultiLineString ((0 0, 1 1),(1 1, 2 2))'

Further reading: Explode lines algorithm

13.2.13.110. shared_paths

Returns a collection containing paths shared by the two input geometries. Those going in the same direction are in the first element of the collection, those going in the opposite direction are in the second element. The paths themselves are given in the direction of the first geometry.

语法	shared_paths(geometry1, geometry2)
参数	geometry1 - a LineString/MultiLineString geometry geometry2 - a LineString/MultiLineString geometry
Examples	geom_to_wkt(shared_paths(geom_from_wkt('MULTILINESTRING((26 125,26 200,126 200,126 125,26 125),(51 150,101 150,76 175,51 150)))'),geom_from_wkt('LINESTRING(151 100,126 156.25,126 125,90 161, 76 175)'))) → 'GeometryCollection (MultiLineString ((126 156.25, 126 125),(101 150, 90 161),(90 161, 76 175)),MultiLineString EMPTY)' geom_to_wkt(shared_paths(geom_from_wkt('LINESTRING(76 175,90 161,126 125,126 156.25,151 100)'),geom_from_wkt('MULTILINESTRING((26 125,26 200,126 200,126 125,26 125),(51 150,101 150,76 175,51 150))'))) → 'GeometryCollection (MultiLineString EMPTY,MultiLineString ((76 175, 90 161),(90 161, 101 150),(126 125, 126 156.25)))'

13.2.13.111. shortest_line

Returns the shortest line joining geometry1 to geometry2. The resultant line will start at geometry1 and end at geometry2.

语法	shortest_line(geometry1, geometry2)
参数	geometry1 - geometry to find shortest line from geometry2 - geometry to find shortest line to
Examples	geom_to_wkt(shortest_line(geom_from_wkt('LINESTRING (20 80, 98 190, 110 180, 50 75 )'),geom_from_wkt('POINT(100 100)'))) → 'LineString(73.0769 115.384, 100 100)'

13.2.13.112. simplify

Simplifies a geometry by removing nodes using a distance based threshold (ie, the Douglas Peucker algorithm). The algorithm preserves large deviations in geometries and reduces the number of vertices in nearly straight segments.

语法	simplify(geometry, tolerance)
参数	geometry - a geometry tolerance - maximum deviation from straight segments for points to be removed
Examples	geom_to_wkt(simplify(geometry:=geom_from_wkt('LineString(0 0, 5 0.1, 10 0)'),tolerance:=5)) → 'LineString(0 0, 10 0)'

Further reading: Simplify algorithm

13.2.13.113. simplify_vw

Simplifies a geometry by removing nodes using an area based threshold (ie, the Visvalingam-Whyatt algorithm). The algorithm removes vertices which create small areas in geometries, e.g., narrow spikes or nearly straight segments.

语法	simplify_vw(geometry, tolerance)
参数	geometry - a geometry tolerance - a measure of the maximum area created by a node for the node to be removed
Examples	geom_to_wkt(simplify_vw(geometry:=geom_from_wkt('LineString(0 0, 5 0, 5.01 10, 5.02 0, 10 0)'),tolerance:=5)) → 'LineString(0 0, 10 0)'

Further reading: Simplify algorithm

13.2.13.114. single_sided_buffer

Returns a geometry formed by buffering out just one side of a linestring geometry. Distances are in the Spatial Reference System of this geometry.

语法	single_sided_buffer(geometry, distance, [segments=8], [join=1], [miter_limit=2.0]) [] 标记可选参数
参数	geometry - a (multi)linestring geometry distance - buffer distance. Positive values will be buffered to the left of lines, negative values to the right segments - number of segments to use to represent a quarter circle when a round join style is used. A larger number results in a smoother buffer with more nodes. join - join style for corners, where 1 = round, 2 = miter and 3 = bevel miter_limit - limit on the miter ratio used for very sharp corners (when using miter joins only)
Examples	single_sided_buffer($geometry, 10.5) → line buffered to the left by 10.5 units single_sided_buffer($geometry, -10.5) → line buffered to the right by 10.5 units single_sided_buffer($geometry, 10.5, segments:=16, join:=1) → line buffered to the left by 10.5 units, using more segments to result in a smoother buffer single_sided_buffer($geometry, 10.5, join:=3) → line buffered to the left by 10.5 units, using a beveled join

Further reading: Single sided buffer algorithm

13.2.13.115. sinuosity

Returns the sinuosity of a curve, which is the ratio of the curve length to the straight (2D) distance between its endpoints.

语法	sinuosity(geometry)
参数	geometry - Input curve (circularstring, linestring)
Examples	round(sinuosity(geom_from_wkt('LINESTRING(2 0, 2 2, 3 2, 3 3)')), 3) → 1.265 sinuosity(geom_from_wkt('LINESTRING( 3 1, 5 1)')) → 1.0

13.2.13.116. smooth

Smooths a geometry by adding extra nodes which round off corners in the geometry. If input geometries contain Z or M values, these will also be smoothed and the output geometry will retain the same dimensionality as the input geometry.

语法	smooth(geometry, [iterations=1], [offset=0.25], [min_length=-1], [max_angle=180]) [] 标记可选参数
参数	geometry - a geometry iterations - number of smoothing iterations to apply. Larger numbers result in smoother but more complex geometries. offset - value between 0 and 0.5 which controls how tightly the smoothed geometry follow the original geometry. Smaller values result in a tighter smoothing, larger values result in looser smoothing. min_length - minimum length of segments to apply smoothing to. This parameter can be used to avoid placing excessive additional nodes in shorter segments of the geometry. max_angle - maximum angle at node for smoothing to be applied (0-180). By lowering the maximum angle intentionally sharp corners in the geometry can be preserved. For instance, a value of 80 degrees will retain right angles in the geometry.
Examples	geom_to_wkt(smooth(geometry:=geom_from_wkt('LineString(0 0, 5 0, 5 5)'),iterations:=1,offset:=0.2,min_length:=-1,max_angle:=180)) → 'LineString (0 0, 4 0, 5 1, 5 5)'

Further reading: Smooth algorithm

13.2.13.117. square_wave

Constructs square/rectangular waves along the boundary of a geometry.

语法	square_wave(geometry, wavelength, amplitude, [strict=False]) [] 标记可选参数
参数	geometry - a geometry wavelength - wavelength of square waveform amplitude - amplitude of square waveform strict - By default the wavelength argument is treated as a "maximum wavelength", where the actual wavelength will be dynamically adjusted so that an exact number of square waves are created along the boundaries of the geometry. If the strict argument is set to true then the wavelength will be used exactly and an incomplete pattern may be used for the final waveform.
Examples	square_wave(geom_from_wkt('LineString(0 0, 10 0)'), 3, 1) → Square waves with wavelength 3 and amplitude 1 along the linestring

13.2.13.118. square_wave_randomized

Constructs randomized square/rectangular waves along the boundary of a geometry.

语法	square_wave_randomized(geometry, min_wavelength, max_wavelength, min_amplitude, max_amplitude, [seed=0]) [] 标记可选参数
参数	geometry - a geometry min_wavelength - minimum wavelength of waves max_wavelength - maximum wavelength of waves min_amplitude - minimum amplitude of waves max_amplitude - maximum amplitude of waves seed - specifies a random seed for generating waves. If the seed is 0, then a completely random set of waves will be generated.
Examples	square_wave_randomized(geom_from_wkt('LineString(0 0, 10 0)'), 2, 3, 0.1, 0.2) → Randomly sized square waves with wavelengths between 2 and 3 and amplitudes between 0.1 and 0.2 along the linestring

13.2.13.119. start_point

Returns the first node from a geometry.

语法	start_point(geometry)
参数	geometry - geometry object
Examples	geom_to_wkt(start_point(geom_from_wkt('LINESTRING(4 0, 4 2, 0 2)'))) → 'Point (4 0)'

Further reading: Extract specific vertices algorithm

13.2.13.120. straight_distance_2d

Returns the direct/euclidean distance between the first and last vertex of a geometry. The geometry must be a curve (circularstring, linestring).

语法	straight_distance_2d(geometry)
参数	geometry - The geometry.
Examples	straight_distance_2d(geom_from_wkt('LINESTRING(1 0, 1 1)')) → 1 round(straight_distance_2d(geom_from_wkt('LINESTRING(1 4, 3 5, 5 0)')), 3) → 5.657

Further reading: length

13.2.13.121. sym_difference

Returns a geometry that represents the portions of two geometries that do not intersect.

语法	sym_difference(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	geom_to_wkt( sym_difference( geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ), geom_from_wkt( 'LINESTRING(3 3, 8 8)' ) ) ) → 'LINESTRING(5 5, 8 8)'

Further reading: Symmetrical difference algorithm

13.2.13.122. tapered_buffer

Creates a buffer along a line geometry where the buffer diameter varies evenly over the length of the line.

语法	tapered_buffer(geometry, start_width, end_width, [segments=8]) [] 标记可选参数
参数	geometry - input geometry. Must be a (multi)line geometry. start_width - width of buffer at start of line, end_width - width of buffer at end of line. segments - number of segments to approximate quarter-circle curves in the buffer.
Examples	tapered_buffer(geometry:=geom_from_wkt('LINESTRING(1 2, 4 2)'),start_width:=1,end_width:=2,segments:=8) → A tapered buffer starting with a diameter of 1 and ending with a diameter of 2 along the linestring geometry.

图 13.8 Tapered buffer on line features

Further reading: Tapered buffers algorithm

13.2.13.123. touches

Tests whether a geometry touches another. Returns TRUE if the geometries have at least one point in common, but their interiors do not intersect.

语法	touches(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	touches( geom_from_wkt( 'LINESTRING(5 3, 4 4)' ), geom_from_wkt( 'LINESTRING(3 3, 4 4, 5 5)' ) ) → TRUE touches( geom_from_wkt( 'POINT(4 4)' ), geom_from_wkt( 'POINT(5 5)' ) ) → FALSE

Further reading: overlay_touches

13.2.13.124. transform

Returns the geometry transformed from a source CRS to a destination CRS.

语法	transform(geometry, source_auth_id, dest_auth_id)
参数	geometry - a geometry source_auth_id - the source auth CRS ID dest_auth_id - the destination auth CRS ID
Examples	geom_to_wkt( transform( make_point(488995.53240249, 7104473.38600835), 'EPSG:2154', 'EPSG:4326' ) ) → 'POINT(0 51)'

Further reading: Reproject layer algorithm

13.2.13.125. translate

Returns a translated version of a geometry. Calculations are in the Spatial Reference System of this geometry.

语法	translate(geometry, dx, dy)
参数	geometry - a geometry dx - delta x dy - delta y
Examples	translate($geometry, 5, 10) → a geometry of the same type like the original one

图 13.9 Translating features

Further reading: Translate algorithm

13.2.13.126. triangular_wave

Constructs triangular waves along the boundary of a geometry.

语法	triangular_wave(geometry, wavelength, amplitude, [strict=False]) [] 标记可选参数
参数	geometry - a geometry wavelength - wavelength of triangular waveform amplitude - amplitude of triangular waveform strict - By default the wavelength argument is treated as a "maximum wavelength", where the actual wavelength will be dynamically adjusted so that an exact number of triangular waves are created along the boundaries of the geometry. If the strict argument is set to true then the wavelength will be used exactly and an incomplete pattern may be used for the final waveform.
Examples	triangular_wave(geom_from_wkt('LineString(0 0, 10 0)'), 3, 1) → Triangular waves with wavelength 3 and amplitude 1 along the linestring

13.2.13.127. triangular_wave_randomized

Constructs randomized triangular waves along the boundary of a geometry.

语法	triangular_wave_randomized(geometry, min_wavelength, max_wavelength, min_amplitude, max_amplitude, [seed=0]) [] 标记可选参数
参数	geometry - a geometry min_wavelength - minimum wavelength of waves max_wavelength - maximum wavelength of waves min_amplitude - minimum amplitude of waves max_amplitude - maximum amplitude of waves seed - specifies a random seed for generating waves. If the seed is 0, then a completely random set of waves will be generated.
Examples	triangular_wave_randomized(geom_from_wkt('LineString(0 0, 10 0)'), 2, 3, 0.1, 0.2) → Randomly sized triangular waves with wavelengths between 2 and 3 and amplitudes between 0.1 and 0.2 along the linestring

13.2.13.128. union

Returns a geometry that represents the point set union of the geometries.

语法	union(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	geom_to_wkt( union( make_point(4, 4), make_point(5, 5) ) ) → 'MULTIPOINT(4 4, 5 5)'

13.2.13.129. wave

Constructs rounded (sine-like) waves along the boundary of a geometry.

语法	wave(geometry, wavelength, amplitude, [strict=False]) [] 标记可选参数
参数	geometry - a geometry wavelength - wavelength of sine-like waveform amplitude - amplitude of sine-like waveform strict - By default the wavelength argument is treated as a "maximum wavelength", where the actual wavelength will be dynamically adjusted so that an exact number of waves are created along the boundaries of the geometry. If the strict argument is set to true then the wavelength will be used exactly and an incomplete pattern may be used for the final waveform.
Examples	wave(geom_from_wkt('LineString(0 0, 10 0)'), 3, 1) → Sine-like waves with wavelength 3 and amplitude 1 along the linestring

13.2.13.130. wave_randomized

Constructs randomized curved (sine-like) waves along the boundary of a geometry.

语法	wave_randomized(geometry, min_wavelength, max_wavelength, min_amplitude, max_amplitude, [seed=0]) [] 标记可选参数
参数	geometry - a geometry min_wavelength - minimum wavelength of waves max_wavelength - maximum wavelength of waves min_amplitude - minimum amplitude of waves max_amplitude - maximum amplitude of waves seed - specifies a random seed for generating waves. If the seed is 0, then a completely random set of waves will be generated.
Examples	wave_randomized(geom_from_wkt('LineString(0 0, 10 0)'), 2, 3, 0.1, 0.2) → Randomly sized curved waves with wavelengths between 2 and 3 and amplitudes between 0.1 and 0.2 along the linestring

13.2.13.131. wedge_buffer

Returns a wedge shaped buffer originating from a point geometry.

语法	wedge_buffer(center, azimuth, width, outer_radius, [inner_radius=0.0]) [] 标记可选参数
参数	center - center point (origin) of buffer. Must be a point geometry. azimuth - angle (in degrees) for the middle of the wedge to point. width - buffer width (in degrees). Note that the wedge will extend to half of the angular width either side of the azimuth direction. outer_radius - outer radius for buffers inner_radius - optional inner radius for buffers
Examples	wedge_buffer(center:=geom_from_wkt('POINT(1 2)'),azimuth:=90,width:=180,outer_radius:=1) → A wedge shaped buffer centered on the point (1,2), facing to the East, with a width of 180 degrees and outer radius of 1.

Further reading: Create wedge buffers algorithm

13.2.13.132. within

Tests whether a geometry is within another. Returns TRUE if the geometry1 is completely within geometry2.

语法	within(geometry1, geometry2)
参数	geometry1 - a geometry geometry2 - a geometry
Examples	within( geom_from_wkt( 'POINT( 0.5 0.5)' ), geom_from_wkt( 'POLYGON((0 0, 0 1, 1 1, 1 0, 0 0))' ) ) → TRUE within( geom_from_wkt( 'POINT( 5 5 )' ), geom_from_wkt( 'POLYGON((0 0, 0 1, 1 1, 1 0, 0 0 ))' ) ) → FALSE

Further reading: overlay_within

13.2.13.133. $x

Returns the x coordinate of the current point feature. If the feature is a multipoint feature, then the x-coordinate of the first point will be returned.

语法	$x
Examples	$x → 42

13.2.13.134. x

Returns the x coordinate of a point geometry, or the x coordinate of the centroid for a non-point geometry.

语法	x(geometry)
参数	geometry - a geometry
Examples	x( geom_from_wkt( 'POINT(2 5)' ) ) → 2 x( $geometry ) → x coordinate of the current feature's centroid

13.2.13.135. $x_at

Retrieves a x coordinate of the current feature's geometry.

语法	$x_at(i)
参数	i - index of point of a line (indices start at 0; negative values apply from the last index, starting at -1)
Examples	$x_at(1) → 5

13.2.13.136. x_max

Returns the maximum x coordinate of a geometry. Calculations are in the spatial reference system of this geometry.

语法	x_max(geometry)
参数	geometry - a geometry
Examples	x_max( geom_from_wkt( 'LINESTRING(2 5, 3 6, 4 8)') ) → 4

13.2.13.137. x_min

Returns the minimum x coordinate of a geometry. Calculations are in the spatial reference system of this geometry.

语法	x_min(geometry)
参数	geometry - a geometry
Examples	x_min( geom_from_wkt( 'LINESTRING(2 5, 3 6, 4 8)') ) → 2

13.2.13.138. $y

Returns the y coordinate of the current point feature. If the feature is a multipoint feature, then the y-coordinate of the first point will be returned.

语法	$y
Examples	$y → 42

13.2.13.139. y

Returns the y coordinate of a point geometry, or the y coordinate of the centroid for a non-point geometry.

语法	y(geometry)
参数	geometry - a geometry
Examples	y( geom_from_wkt( 'POINT(2 5)' ) ) → 5 y( $geometry ) → y coordinate of the current feature's centroid

13.2.13.140. $y_at

Retrieves a y coordinate of the current feature's geometry.

语法	$y_at(i)
参数	i - index of point of a line (indices start at 0; negative values apply from the last index, starting at -1)
Examples	$y_at(1) → 2

13.2.13.141. y_max

Returns the maximum y coordinate of a geometry. Calculations are in the spatial reference system of this geometry.

语法	y_max(geometry)
参数	geometry - a geometry
Examples	y_max( geom_from_wkt( 'LINESTRING(2 5, 3 6, 4 8)') ) → 8

13.2.13.142. y_min

Returns the minimum y coordinate of a geometry. Calculations are in the spatial reference system of this geometry.

语法	y_min(geometry)
参数	geometry - a geometry
Examples	y_min( geom_from_wkt( 'LINESTRING(2 5, 3 6, 4 8)') ) → 5

13.2.13.143. $z

Returns the z value of the current point feature if it is 3D. If the feature is a multipoint feature, then the z value of the first point will be returned.

语法	$z
Examples	$z → 123

13.2.13.144. z

Returns the z coordinate of a point geometry, or NULL if the geometry has no z value.

语法	z(geometry)
参数	geometry - a point geometry
Examples	z( geom_from_wkt( 'POINTZ(2 5 7)' ) ) → 7

13.2.13.145. z_max

Returns the maximum z coordinate of a geometry, or NULL if the geometry has no z value.

语法	z_max(geometry)
参数	geometry - a geometry with z coordinate
Examples	z_max( geom_from_wkt( 'POINT ( 0 0 1 )' ) ) → 1 z_max( geom_from_wkt( 'MULTIPOINT ( 0 0 1 , 1 1 3 )' ) ) → 3 z_max( make_line( make_point( 0,0,0 ), make_point( -1,-1,-2 ) ) ) → 0 z_max( geom_from_wkt( 'LINESTRING( 0 0 0, 1 0 2, 1 1 -1 )' ) ) → 2 z_max( geom_from_wkt( 'POINT ( 0 0 )' ) ) → NULL

13.2.13.146. z_min

Returns the minimum z coordinate of a geometry, or NULL if the geometry has no z value.

语法	z_min(geometry)
参数	geometry - a geometry with z coordinate
Examples	z_min( geom_from_wkt( 'POINT ( 0 0 1 )' ) ) → 1 z_min( geom_from_wkt( 'MULTIPOINT ( 0 0 1 , 1 1 3 )' ) ) → 1 z_min( make_line( make_point( 0,0,0 ), make_point( -1,-1,-2 ) ) ) → -2 z_min( geom_from_wkt( 'LINESTRING( 0 0 0, 1 0 2, 1 1 -1 )' ) ) → -1 z_min( geom_from_wkt( 'POINT ( 0 0 )' ) ) → NULL

13.2.14. Layout Functions

This group contains functions to manipulate print layout items properties.

13.2.14.1. item_variables

Returns a map of variables from a layout item inside this print layout.

语法	item_variables(id)
参数	id - layout item ID
Examples	map_get( item_variables('Map 0'), 'map_scale') → scale of the item 'Map 0' in the current print layout

Further reading: List of default variables

13.2.14.2. map_credits

Returns a list of credit (usage rights) strings for the layers shown in a layout map item.

语法	map_credits(id, [include_layer_names=false], [layer_name_separator=': ']) [] 标记可选参数
参数	id - map item ID include_layer_names - Set to true to include layer names before their credit strings layer_name_separator - String to insert between layer names and their credit strings, if include_layer_names is true
Examples	array_to_string( map_credits( 'Main Map' ) ) → comma separated list of layer credits for layers shown in the 'Main Map' layout item, e.g 'CC-BY-NC, CC-BY-SA' array_to_string( map_credits( 'Main Map', include_layer_names := true, layer_name_separator := ': ' ) ) → comma separated list of layer names and their credits for layers shown in the 'Main Map' layout item, e.g. 'Railway lines: CC-BY-NC, Basemap: CC-BY-SA'

This function requires the Access metadata properties of the layers to have been filled.

13.2.15. Map Layers

This group contains a list of the available layers in the current project and, for each layer, their fields (stored in the dataset, virtual or auxiliary ones as well as from joins). The fields can be interacted the same way as mentioned in Fields and Values, except that a double-click will add the name as a string (single quoted) to the expression instead of as a field reference given that they do not belong to the active layer. This offers a convenient way to write expressions referring to different layers, such as when performing aggregates, attribute or spatial queries.

It also provides some convenient functions to manipulate layers.

13.2.15.1. decode_uri

Takes a layer and decodes the uri of the underlying data provider. It depends on the dataprovider, which data is available.

语法	decode_uri(layer, [part]) [] 标记可选参数
参数	layer - The layer for which the uri should be decoded. part - The part of the uri to return. If unspecified, a map with all uri parts will be returned.
Examples	decode_uri(@layer) → {'layerId': '0', 'layerName': '', 'path': '/home/qgis/shapefile.shp'} decode_uri(@layer) → {'layerId': NULL, 'layerName': 'layer', 'path': '/home/qgis/geopackage.gpkg'} decode_uri(@layer, 'path') → 'C:\my_data\qgis\shape.shp'

13.2.15.2. layer_property

Returns a matching layer property or metadata value.

语法	layer_property(layer, property)
参数	图层 - 字符串, 表示图层名称或图层ID property - a string corresponding to the property to return. Valid options are: name: layer name id: layer ID title: metadata title string abstract: metadata abstract string keywords: metadata keywords data_url: metadata URL attribution: metadata attribution string attribution_url: metadata attribution URL source: layer source min_scale: minimum display scale for layer max_scale: maximum display scale for layer is_editable: if layer is in edit mode crs: layer CRS crs_definition: layer CRS full definition crs_description: layer CRS description extent: layer extent (as a geometry object) distance_units: layer distance units type: layer type, e.g., Vector or Raster storage_type: storage format (vector layers only) geometry_type: geometry type, e.g., Point (vector layers only) feature_count: approximate feature count for layer (vector layers only) path: File path to the layer data source. Only available for file based layers.
Examples	layer_property('streets','title') → 'Basemap Streets' layer_property('airports','feature_count') → 120 layer_property('landsat','crs') → 'EPSG:4326'

Further reading: vector, raster and mesh layer properties

13.2.16. Maps Functions

This group contains functions to create or manipulate keys and values of map data structures (also known as dictionary objects, key-value pairs, or associative arrays). Unlike the list data structure where values order matters, the order of the key-value pairs in the map object is not relevant and values are identified by their keys.

13.2.16.1. from_json

Loads a JSON formatted string.

语法	from_json(string)
参数	string - JSON string
Examples	from_json('{"qgis":"rocks"}') → { 'qgis': 'rocks' } from_json('[1,2,3]') → [1,2,3]

13.2.16.2. hstore_to_map

Creates a map from a hstore-formatted string.

语法	hstore_to_map(string)
参数	string - the input string
Examples	hstore_to_map('qgis=>rocks') → { 'qgis': 'rocks' }

13.2.16.3. map

Returns a map containing all the keys and values passed as pair of parameters.

语法	map(key1, value1, key2, value2, ...)
参数	key - a key (string) value - a value
Examples	map('1','one','2', 'two') → { '1': 'one', '2': 'two' } map('1','one','2', 'two')['1'] → 'one'

13.2.16.4. map_akeys

Returns all the keys of a map as an array.

语法	map_akeys(map)
参数	map - a map
Examples	map_akeys(map('1','one','2','two')) → [ '1', '2' ]

13.2.16.5. map_avals

Returns all the values of a map as an array.

语法	map_avals(map)
参数	map - a map
Examples	map_avals(map('1','one','2','two')) → [ 'one', 'two' ]

13.2.16.6. map_concat

Returns a map containing all the entries of the given maps. If two maps contain the same key, the value of the second map is taken.

语法	map_concat(map1, map2, ...)
参数	map - a map
Examples	map_concat(map('1','one', '2','overridden'),map('2','two', '3','three')) → { '1': 'one', '2': 'two', '3': 'three' }

13.2.16.7. map_delete

Returns a map with the given key and its corresponding value deleted.

语法	map_delete(map, key)
参数	map - a map key - the key to delete
Examples	map_delete(map('1','one','2','two'),'2') → { '1': 'one' }

13.2.16.8. map_exist

Returns TRUE if the given key exists in the map.

语法	map_exist(map, key)
参数	map - a map key - the key to lookup
Examples	map_exist(map('1','one','2','two'),'3') → FALSE

13.2.16.9. map_get

Returns the value of a map, given its key. Returns NULL if the key does not exist.

语法	map_get(map, key)
参数	map - a map key - the key to lookup
Examples	map_get(map('1','one','2','two'),'2') → 'two' map_get( item_variables('Map 0'), 'map_scale') → scale of the item 'Map 0' (if it exists) in the current print layout

提示

You can also use the index operator ([]) to get a value from a map.

13.2.16.10. map_insert

Returns a map with an added key/value. If the key already exists, its value is overridden.

语法	map_insert(map, key, value)
参数	map - a map key - the key to add value - the value to add
Examples	map_insert(map('1','one'),'3','three') → { '1': 'one', '3': 'three' } map_insert(map('1','one','2','overridden'),'2','two') → { '1': 'one', '2': 'two' }

13.2.16.11. map_prefix_keys

Returns a map with all keys prefixed by a given string.

语法	map_prefix_keys(map, prefix)
参数	map - a map prefix - a string
Examples	map_prefix_keys(map('1','one','2','two'), 'prefix-') → { 'prefix-1': 'one', 'prefix-2': 'two' }

13.2.16.12. map_to_hstore

Merge map elements into a hstore-formatted string.

语法	map_to_hstore(map)
参数	map - the input map
Examples	map_to_hstore(map('qgis','rocks')) → '"qgis"=>"rocks"'

13.2.16.13. to_json

Create a JSON formatted string from a map, array or other value.

语法	to_json(value)
参数	value - The input value
Examples	to_json(map('qgis','rocks')) → {"qgis":"rocks"} to_json(array(1,2,3)) → [1,2,3]

13.2.16.14. url_encode

Returns an URL encoded string from a map. Transforms all characters in their properly-encoded form producing a fully-compliant query string.

Note that the plus sign '+' is not converted.

语法	url_encode(map)
参数	map - a map.
Examples	url_encode(map('a&+b', 'a and plus b', 'a=b', 'a equals b')) → 'a%26+b=a%20and%20plus%20b&a%3Db=a%20equals%20b'

13.2.17. Mathematical Functions

This group contains math functions (e.g., square root, sin and cos).

13.2.17.1. abs

Returns the absolute value of a number.

语法	abs(value)
参数	value - a number
Examples	abs(-2) → 2

13.2.17.2. acos

Returns the inverse cosine of a value in radians.

语法	acos(value)
参数	value - cosine of an angle in radians
Examples	acos(0.5) → 1.0471975511966

13.2.17.3. asin

Returns the inverse sine of a value in radians.

语法	asin(value)
参数	value - sine of an angle in radians
Examples	asin(1.0) → 1.5707963267949

13.2.17.4. atan

Returns the inverse tangent of a value in radians.

语法	atan(value)
参数	value - tan of an angle in radians
Examples	atan(0.5) → 0.463647609000806

13.2.17.5. atan2

Returns the inverse tangent of dy/dx by using the signs of the two arguments to determine the quadrant of the result.

语法	atan2(dy, dx)
参数	dy - y coordinate difference dx - x coordinate difference
Examples	atan2(1.0, 1.732) → 0.523611477769969

13.2.17.6. ceil

Rounds a number upwards.

语法	ceil(value)
参数	value - a number
Examples	ceil(4.9) → 5 ceil(-4.9) → -4

13.2.17.7. clamp

Restricts an input value to a specified range.

语法	clamp(minimum, input, maximum)
参数	minimum - the smallest value input is allowed to take. input - a value which will be restricted to the range specified by minimum and maximum maximum - the largest value input is allowed to take
Examples	clamp(1,5,10) → 5 input is between 1 and 10 so is returned unchanged clamp(1,0,10) → 1 input is less than minimum value of 1, so function returns 1 clamp(1,11,10) → 10 input is greater than maximum value of 10, so function returns 10

13.2.17.8. cos

Returns cosine of an angle.

语法	cos(angle)
参数	angle - angle in radians
Examples	cos(1.571) → 0.000796326710733263

13.2.17.9. degrees

Converts from radians to degrees.

语法	degrees(radians)
参数	radians - numeric value
Examples	degrees(3.14159) → 180 degrees(1) → 57.2958

13.2.17.10. exp

Returns exponential of an value.

语法	exp(value)
参数	value - number to return exponent of
Examples	exp(1.0) → 2.71828182845905

13.2.17.11. floor

Rounds a number downwards.

语法	floor(value)
参数	value - a number
Examples	floor(4.9) → 4 floor(-4.9) → -5

13.2.17.12. ln

Returns the natural logarithm of a value.

语法	ln(value)
参数	value - numeric value
Examples	ln(1) → 0 ln(2.7182818284590452354) → 1

13.2.17.13. log

Returns the value of the logarithm of the passed value and base.

语法	log(base, value)
参数	base - any positive number value - any positive number
Examples	log(2, 32) → 5 log(0.5, 32) → -5

13.2.17.14. log10

Returns the value of the base 10 logarithm of the passed expression.

语法	log10(value)
参数	value - any positive number
Examples	log10(1) → 0 log10(100) → 2

13.2.17.15. max

Returns the largest value in a set of values.

语法	max(value1, value2, ...)
参数	value - a number
Examples	max(2,10.2,5.5) → 10.2 max(20.5,NULL,6.2) → 20.5

13.2.17.16. min

Returns the smallest value in a set of values.

语法	min(value1, value2, ...)
参数	value - a number
Examples	min(20.5,10,6.2) → 6.2 min(2,-10.3,NULL) → -10.3

13.2.17.17. pi

Returns value of pi for calculations.

语法	pi()
Examples	pi() → 3.14159265358979

13.2.17.18. radians

Converts from degrees to radians.

语法	radians(degrees)
参数	degrees - numeric value
Examples	radians(180) → 3.14159 radians(57.2958) → 1

13.2.17.19. rand

Returns a random integer within the range specified by the minimum and maximum argument (inclusive). If a seed is provided, the returned will always be the same, depending on the seed.

语法	rand(min, max, [seed=NULL]) [] 标记可选参数
参数	min - an integer representing the smallest possible random number desired max - an integer representing the largest possible random number desired seed - any value to use as seed
Examples	rand(1, 10) → 8

13.2.17.20. randf

Returns a random float within the range specified by the minimum and maximum argument (inclusive). If a seed is provided, the returned will always be the same, depending on the seed.

语法	randf([min=0.0], [max=1.0], [seed=NULL]) [] 标记可选参数
参数	min - an float representing the smallest possible random number desired max - an float representing the largest possible random number desired seed - any value to use as seed
Examples	randf(1, 10) → 4.59258286403147

13.2.17.21. round

Rounds a number to number of decimal places.

语法	round(value, [places=0]) [] 标记可选参数
参数	value - decimal number to be rounded places - Optional integer representing number of places to round decimals to. Can be negative.
Examples	round(1234.567, 2) → 1234.57 round(1234.567) → 1235 round(1234.567, -1) → 1230

13.2.17.22. scale_exp

Transforms a given value from an input domain to an output range using an exponential curve. This function can be used to ease values in or out of the specified output range.

语法	scale_exp(value, domain_min, domain_max, range_min, range_max, exponent)
参数	value - A value in the input domain. The function will return a corresponding scaled value in the output range. domain_min - Specifies the minimum value in the input domain, the smallest value the input value should take. domain_max - Specifies the maximum value in the input domain, the largest value the input value should take. range_min - Specifies the minimum value in the output range, the smallest value which should be output by the function. range_max - Specifies the maximum value in the output range, the largest value which should be output by the function. exponent - A positive value (greater than 0), which dictates the way input values are mapped to the output range. Large exponents will cause the output values to 'ease in', starting slowly before accelerating as the input values approach the domain maximum. Smaller exponents (less than 1) will cause output values to 'ease out', where the mapping starts quickly but slows as it approaches the domain maximum.
Examples	scale_exp(5,0,10,0,100,2) → 25 easing in, using an exponent of 2 scale_exp(3,0,10,0,100,0.5) → 54.772 easing out, using an exponent of 0.5

13.2.17.23. scale_linear

Transforms a given value from an input domain to an output range using linear interpolation.

语法	scale_linear(value, domain_min, domain_max, range_min, range_max)
参数	value - A value in the input domain. The function will return a corresponding scaled value in the output range. domain_min - Specifies the minimum value in the input domain, the smallest value the input value should take. domain_max - Specifies the maximum value in the input domain, the largest value the input value should take. range_min - Specifies the minimum value in the output range, the smallest value which should be output by the function. range_max - Specifies the maximum value in the output range, the largest value which should be output by the function.
Examples	scale_linear(5,0,10,0,100) → 50 scale_linear(0.2,0,1,0,360) → 72 scaling a value between 0 and 1 to an angle between 0 and 360 scale_linear(1500,1000,10000,9,20) → 9.6111111 scaling a population which varies between 1000 and 10000 to a font size between 9 and 20

13.2.17.24. sin

Returns the sine of an angle.

语法	sin(angle)
参数	angle - angle in radians
Examples	sin(1.571) → 0.999999682931835

13.2.17.25. sqrt

Returns square root of a value.

语法	sqrt(value)
参数	value - a number
Examples	sqrt(9) → 3

13.2.17.26. tan

Returns the tangent of an angle.

语法	tan(angle)
参数	angle - angle in radians
Examples	tan(1.0) → 1.5574077246549

13.2.18. Meshes Functions

This group contains functions which calculate or return mesh related values.

13.2.18.1. $face_area

Returns the area of the current mesh face. The area calculated by this function respects both the current project's ellipsoid setting and area unit settings. For example, if an ellipsoid has been set for the project then the calculated area will be ellipsoidal, and if no ellipsoid is set then the calculated area will be planimetric.

语法	$face_area
Examples	$face_area → 42

13.2.18.2. $face_index

Returns the index of the current mesh face.

语法	$face_index
Examples	$face_index → 4581

13.2.18.3. $vertex_as_point

Returns the current vertex as a point geometry.

语法	$vertex_as_point
Examples	geom_to_wkt( $vertex_as_point ) → 'POINT(800 1500 41)'

13.2.18.4. $vertex_index

Returns the index of the current mesh vertex.

语法	$vertex_index
Examples	$vertex_index → 9874

13.2.18.5. $vertex_x

Returns the X coordinate of the current mesh vertex.

语法	$vertex_x
Examples	$vertex_x → 42.12

13.2.18.6. $vertex_y

Returns the Y coordinate of the current mesh vertex.

语法	$vertex_y
Examples	$vertex_y → 12.24

13.2.18.7. $vertex_z

Returns the Z value of the current mesh vertex.

语法	$vertex_z
Examples	$vertex_z → 42

13.2.19. Operators

This group contains operators (e.g., +, -, *). Note that for most of the mathematical functions below, if one of the inputs is NULL then the result is NULL.

13.2.19.1. %

Remainder of division. Takes the sign of the dividend.

语法	a % b
参数	a - value b - value
Examples	9 % 2 → 1 9 % -2 → 1 -9 % 2 → -1 5 % NULL → NULL

13.2.19.2. *

Multiplication of two values

语法	a * b
参数	a - value b - value
Examples	5 * 4 → 20 5 * NULL → NULL

13.2.19.3. +

Addition of two values. If one of the values is NULL the result will be NULL.

语法	a + b
参数	a - value b - value
Examples	5 + 4 → 9 5 + NULL → NULL 'QGIS ' + 'ROCKS' → 'QGIS ROCKS' to_datetime('2020-08-01 12:00:00') + '1 day 2 hours' → 2020-08-02T14:00:00

Further reading: concat, ||

13.2.19.4. -

Subtraction of two values. If one of the values is NULL the result will be NULL.

语法	a - b
参数	a - value b - value
Examples	5 - 4 → 1 5 - NULL → NULL to_datetime('2012-05-05 12:00:00') - to_interval('1 day 2 hours') → 2012-05-04T10:00:00

13.2.19.5. /

Division of two values

语法	a / b
参数	a - value b - value
Examples	5 / 4 → 1.25 5 / NULL → NULL

13.2.19.6. <

Compares two values and evaluates to 1 if the left value is less than the right value.

语法	a < b
参数	a - value b - value
Examples	5 < 4 → FALSE 5 < 5 → FALSE 4 < 5 → TRUE

13.2.19.7. <=

Compares two values and evaluates to 1 if the left value is less or equal than the right value.

语法	a <= b
参数	a - value b - value
Examples	5 <= 4 → FALSE 5 <= 5 → TRUE 4 <= 5 → TRUE

13.2.19.8. <>

Compares two values and evaluates to 1 if they are not equal.

语法	a <> b
参数	a - value b - value
Examples	5 <> 4 → TRUE 4 <> 4 → FALSE 5 <> NULL → NULL NULL <> NULL → NULL

13.2.19.9. =

Compares two values and evaluates to 1 if they are equal.

语法	a = b
参数	a - value b - value
Examples	5 = 4 → FALSE 4 = 4 → TRUE 5 = NULL → NULL NULL = NULL → NULL

13.2.19.10. >

Compares two values and evaluates to 1 if the left value is greater than the right value.

语法	a > b
参数	a - value b - value
Examples	5 > 4 → TRUE 5 > 5 → FALSE 4 > 5 → FALSE

13.2.19.11. >=

Compares two values and evaluates to 1 if the left value is greater or equal than the right value.

语法	a >= b
参数	a - value b - value
Examples	5 >= 4 → TRUE 5 >= 5 → TRUE 4 >= 5 → FALSE

13.2.19.12. AND

Returns TRUE when conditions a and b are true.

语法	a AND b
参数	a - condition b - condition
Examples	TRUE AND TRUE → TRUE TRUE AND FALSE → FALSE 4 = 2+2 AND 1 = 1 → TRUE 4 = 2+2 AND 1 = 2 → FALSE

13.2.19.13. BETWEEN

Returns TRUE if value is within the specified range. The range is considered inclusive of the bounds. To test for exclusion NOT BETWEEN can be used.

语法	value BETWEEN lower_bound AND higher_bound
参数	value - the value to compare with a range. It can be a string, a number or a date. lower_bound AND higher_bound - range bounds
Examples	'B' BETWEEN 'A' AND 'C' → TRUE 2 BETWEEN 1 AND 3 → TRUE 2 BETWEEN 2 AND 3 → TRUE 'B' BETWEEN 'a' AND 'c' → FALSE lower('B') BETWEEN 'a' AND 'b' → TRUE

备注

value BETWEEN lower_bound AND higher_bound is the same as "value >= lower_bound AND value <= higher_bound".

Further reading: NOT BETWEEN

13.2.19.14. ILIKE

Returns TRUE if the first parameter matches case-insensitive the supplied pattern. LIKE can be used instead of ILIKE to make the match case-sensitive. Works with numbers also.

语法	string/number ILIKE pattern
参数	string/number - string to search pattern - pattern to find, you can use '%' as a wildcard, '_' as a single char and '\\' to escape these special characters.
Examples	'A' ILIKE 'A' → TRUE 'A' ILIKE 'a' → TRUE 'A' ILIKE 'B' → FALSE 'ABC' ILIKE 'b' → FALSE 'ABC' ILIKE 'B' → FALSE 'ABC' ILIKE '_b_' → TRUE 'ABC' ILIKE '_B_' → TRUE 'ABCD' ILIKE '_b_' → FALSE 'ABCD' ILIKE '_B_' → FALSE 'ABCD' ILIKE '_b%' → TRUE 'ABCD' ILIKE '_B%' → TRUE 'ABCD' ILIKE '%b%' → TRUE 'ABCD' ILIKE '%B%' → TRUE 'ABCD%' ILIKE 'abcd\\%' → TRUE 'ABCD' ILIKE '%B\\%' → FALSE

13.2.19.15. IN

Returns TRUE if value is found within a list of values.

语法	a IN b
参数	a - value b - list of values
Examples	'A' IN ('A','B') → TRUE 'A' IN ('C','B') → FALSE

13.2.19.16. IS

Returns TRUE if a is the same as b.

语法	a IS b
参数	a - any value b - any value
Examples	'A' IS 'A' → TRUE 'A' IS 'a' → FALSE 4 IS 4 → TRUE 4 IS 2+2 → TRUE 4 IS 2 → FALSE $geometry IS NULL → 0, if your geometry is not NULL

13.2.19.17. IS NOT

Returns TRUE if a is not the same as b.

语法	a IS NOT b
参数	a - value b - value
Examples	'a' IS NOT 'b' → TRUE 'a' IS NOT 'a' → FALSE 4 IS NOT 2+2 → FALSE

13.2.19.18. LIKE

Returns TRUE if the first parameter matches the supplied pattern. Works with numbers also.

语法	string/number LIKE pattern
参数	string/number - value pattern - pattern to compare value with, you can use '%' as a wildcard, '_' as a single char and '\\' to escape these special characters.
Examples	'A' LIKE 'A' → TRUE 'A' LIKE 'a' → FALSE 'A' LIKE 'B' → FALSE 'ABC' LIKE 'B' → FALSE 'ABC' LIKE '_B_' → TRUE 'ABCD' LIKE '_B_' → FALSE 'ABCD' LIKE '_B%' → TRUE 'ABCD' LIKE '%B%' → TRUE '1%' LIKE '1\\%' → TRUE '1_' LIKE '1\\%' → FALSE

13.2.19.19. NOT

Negates a condition.

语法	NOT a
参数	a - condition
Examples	NOT 1 → FALSE NOT 0 → TRUE

13.2.19.20. NOT BETWEEN

Returns TRUE if value is not within the specified range. The range is considered inclusive of the bounds.

语法	value NOT BETWEEN lower_bound AND higher_bound
参数	value - the value to compare with a range. It can be a string, a number or a date. lower_bound AND higher_bound - range bounds
Examples	'B' NOT BETWEEN 'A' AND 'C' → FALSE 1.0 NOT BETWEEN 1.1 AND 1.2 → TRUE 2 NOT BETWEEN 2 AND 3 → FALSE 'B' NOT BETWEEN 'a' AND 'c' → TRUE lower('B') NOT BETWEEN 'a' AND 'b' → FALSE

备注

value NOT BETWEEN lower_bound AND higher_bound is the same as "value < lower_bound OR value > higher_bound".

Further reading: BETWEEN

13.2.19.21. OR

Returns TRUE when condition a or b is true.

语法	a OR b
参数	a - condition b - condition
Examples	4 = 2+2 OR 1 = 1 → TRUE 4 = 2+2 OR 1 = 2 → TRUE 4 = 2   OR 1 = 2 → FALSE

13.2.19.22. []

Index operator. Returns an element from an array or map value.

语法	[index]
参数	index - array index or map key value
Examples	array(1,2,3)[0] → 1 array(1,2,3)[2] → 3 array(1,2,3)[-1] → 3 map('a',1,'b',2)['a'] → 1 map('a',1,'b',2)['b'] → 2

Further reading: array_get, map_get

13.2.19.23. ^

Power of two values.

语法	a ^ b
参数	a - value b - value
Examples	5 ^ 4 → 625 5 ^ NULL → NULL

13.2.19.24. ||

Joins two values together into a string.

If one of the values is NULL the result will be NULL. See the CONCAT function for a different behavior.

语法	a || b
参数	a - value b - value
Examples	'Here' || ' and ' || 'there' → 'Here and there' 'Nothing' || NULL → NULL 'Dia: ' || "Diameter" → 'Dia: 25' 1 || 2 → '12'

Further reading: concat, +

13.2.19.25. ~

Performs a regular expression match on a string value. Backslash characters must be double escaped (e.g., "\\s" to match a white space character).

语法	string ~ regex
参数	string - A string value regex - A regular expression. Slashes must be escaped, eg \\d.
Examples	'hello' ~ 'll' → TRUE 'hello' ~ '^ll' → FALSE 'hello' ~ 'llo$' → TRUE 'abc123' ~ '\\d+' → TRUE

Further reading: regexp_match

13.2.20. Processing Functions

This group contains functions that operate on processing algorithms.

13.2.20.1. parameter

Returns the value of a processing algorithm input parameter.

语法	parameter(name)
参数	name - name of the corresponding input parameter
Examples	parameter('BUFFER_SIZE') → 5.6

13.2.21. Rasters Functions

This group contains functions to operate on raster layer.

13.2.21.1. raster_statistic

Returns statistics from a raster layer.

语法	raster_statistic(layer, band, property)
参数	layer - a string, representing either a raster layer name or layer ID band - integer representing the band number from the raster layer, starting at 1 property - a string corresponding to the property to return. Valid options are: min: minimum value max: maximum value avg: average (mean) value stdev: standard deviation of values range: range of values (max - min) sum: sum of all values from raster
Examples	raster_statistic('lc',1,'avg') → Average value from band 1 from 'lc' raster layer raster_statistic('ac2010',3,'min') → Minimum value from band 3 from 'ac2010' raster layer

13.2.21.2. raster_value

Returns the raster value found at the provided point.

语法	raster_value(layer, band, point)
参数	layer - the name or id of a raster layer band - the band number to sample the value from. point - point geometry (for multipart geometries having more than one part, a NULL value will be returned)
Examples	raster_value('dem', 1, make_point(1,1)) → 25

13.2.22. Record and Attributes Functions

This group contains functions that operate on record identifiers.

13.2.22.1. attribute

Returns an attribute from a feature.

Variant 1

Returns the value of an attribute from the current feature.

语法	attribute(attribute_name)
参数	attribute_name - name of attribute to be returned
Examples	attribute( 'name' ) → value stored in 'name' attribute for the current feature

Variant 2

Allows the target feature and attribute name to be specified.

语法	attribute(feature, attribute_name)
参数	feature - a feature attribute_name - name of attribute to be returned
Examples	attribute( @atlas_feature, 'name' ) → value stored in 'name' attribute for the current atlas feature

13.2.22.2. attributes

Returns a map containing all attributes from a feature, with field names as map keys.

Variant 1

Returns a map of all attributes from the current feature.

语法	attributes()
Examples	attributes()['name'] → value stored in 'name' attribute for the current feature

Variant 2

Allows the target feature to be specified.

语法	attributes(feature)
参数	feature - a feature
Examples	attributes( @atlas_feature )['name'] → value stored in 'name' attribute for the current atlas feature

Further reading: Maps Functions

13.2.22.3. $currentfeature

Returns the current feature being evaluated. This can be used with the 'attribute' function to evaluate attribute values from the current feature. WARNING: This function is deprecated. It is recommended to use the replacement @feature variable instead.

语法	$currentfeature
Examples	attribute( $currentfeature, 'name' ) → value stored in 'name' attribute for the current feature

13.2.22.4. display_expression

Returns the display expression for a given feature in a layer. The expression is evaluated by default. Can be used with zero, one or more arguments, see below for details.

No parameters

If called with no parameters, the function will evaluate the display expression of the current feature in the current layer.

语法	display_expression()
Examples	display_expression() → The display expression of the current feature in the current layer.

One 'feature' parameter

If called with a 'feature' parameter only, the function will evaluate the specified feature from the current layer.

语法	display_expression(feature)
参数	feature - The feature which should be evaluated.
Examples	display_expression(@atlas_feature) → The display expression of the current atlas feature.

Layer and feature parameters

If the function is called with both a layer and a feature, it will evaluate the specified feature from the specified layer.

语法	display_expression(layer, feature, [evaluate=true]) [] 标记可选参数
参数	layer - The layer (or its ID or name) feature - The feature which should be evaluated. evaluate - If the expression must be evaluated. If false, the expression will be returned as a string literal only (which could potentially be later evaluated using the 'eval' function).
Examples	display_expression( 'streets', get_feature_by_id('streets', 1)) → The display expression of the feature with the ID 1 on the layer 'streets'. display_expression('a_layer_id', $currentfeature, 'False') → The display expression of the given feature not evaluated.

13.2.22.5. get_feature

Returns the first feature of a layer matching a given attribute value.

Single value variant

Along with the layer ID, a single column and value are specified.

语法	get_feature(layer, attribute, value)
参数	layer - layer name or ID attribute - attribute name to use for the match value - attribute value to match
Examples	get_feature('streets','name','main st') → first feature found in "streets" layer with "main st" value in the "name" field

Map variant

Along with the layer ID, a map containing the columns (key) and their respective value to be used.

语法	get_feature(layer, attribute)
参数	layer - layer name or ID attribute - Map containing the column and value pairs to use
Examples	get_feature('streets',map('name','main st','lane_num','4')) → first feature found in "streets" layer with "main st" value in the "name" field and "4" value in the "lane_num" field

13.2.22.6. get_feature_by_id

Returns the feature with an id on a layer.

语法	get_feature_by_id(layer, feature_id)
参数	layer - layer, layer name or layer id feature_id - the id of the feature which should be returned
Examples	get_feature_by_id('streets', 1) → the feature with the id 1 on the layer "streets"

Further reading: $id

13.2.22.7. $id

Returns the feature id of the current row. WARNING: This function is deprecated. It is recommended to use the replacement @id variable instead.

语法	$id
Examples	$id → 42

13.2.22.8. is_selected

Returns TRUE if a feature is selected. Can be used with zero, one or two arguments, see below for details.

No parameters

If called with no parameters, the function will return TRUE if the current feature in the current layer is selected.

语法	is_selected()
Examples	is_selected() → TRUE if the current feature in the current layer is selected.

One 'feature' parameter

If called with a 'feature' parameter only, the function returns TRUE if the specified feature from the current layer is selected.

语法	is_selected(feature)
参数	feature - The feature which should be checked for selection.
Examples	is_selected(@atlas_feature) → TRUE if the current atlas feature is selected. is_selected(get_feature('streets', 'name', 'Main St.')) → TRUE if the unique named "Main St." feature on the active "streets" layer is selected. is_selected(get_feature_by_id('streets', 1)) → TRUE if the feature with the id 1 on the active "streets" layer is selected.

Two parameters

If the function is called with both a layer and a feature, it will return TRUE if the specified feature from the specified layer is selected.

语法	is_selected(layer, feature)
参数	layer - The layer (its ID or name) on which the selection will be checked. feature - The feature which should be checked for selection.
Examples	is_selected( 'streets', get_feature('streets', 'name', "street_name")) → TRUE if the current building's street is selected (assuming the building layer has a field named 'street_name' and the 'streets' layer has a field called 'name' with unique values). is_selected( 'streets', get_feature_by_id('streets', 1)) → TRUE if the feature with the id 1 on the "streets" layer is selected.

13.2.22.9. maptip

Returns the maptip for a given feature in a layer. The expression is evaluated by default. Can be used with zero, one or more arguments, see below for details.

No parameters

If called with no parameters, the function will evaluate the maptip of the current feature in the current layer.

语法	maptip()
Examples	maptip() → The maptip of the current feature in the current layer.

One 'feature' parameter

If called with a 'feature' parameter only, the function will evaluate the specified feature from the current layer.

语法	maptip(feature)
参数	feature - The feature which should be evaluated.
Examples	maptip(@atlas_feature) → The maptip of the current atlas feature.

Layer and feature parameters

If the function is called with both a layer and a feature, it will evaluate the specified feature from the specified layer.

语法	maptip(layer, feature, [evaluate=true]) [] 标记可选参数
参数	layer - The layer (or its ID or name) feature - The feature which should be evaluated. evaluate - If the expression must be evaluated. If false, the expression will be returned as a string literal only (which could potentially be later evaluated using the 'eval_template' function).
Examples	maptip('streets', get_feature_by_id('streets', 1)) → The maptip of the feature with the ID 1 on the layer 'streets'. maptip('a_layer_id', $currentfeature, 'False') → The maptip of the given feature not evaluated.

13.2.22.10. num_selected

Returns the number of selected features on a given layer. By default works on the layer on which the expression is evaluated.

语法	num_selected([layer=current layer]) [] 标记可选参数
参数	layer - The layer (or its id or name) on which the selection will be checked.
Examples	num_selected() → The number of selected features on the current layer. num_selected('streets') → The number of selected features on the layer streets

13.2.22.11. represent_attributes

Returns a map with the attribute names as keys and the configured representation values as values. The representation value for the attributes depends on the configured widget type for each attribute. Can be used with zero, one or more arguments, see below for details.

No parameters

If called with no parameters, the function will return the representation of the attributes of the current feature in the current layer.

语法	represent_attributes()
Examples	represent_attributes() → The representation of the attributes for the current feature.

One 'feature' parameter

If called with a 'feature' parameter only, the function will return the representation of the attributes of the specified feature from the current layer.

语法	represent_attributes(feature)
参数	feature - The feature which should be evaluated.
Examples	represent_attributes(@atlas_feature) → The representation of the attributes for the specified feature from the current layer.

Layer and feature parameters

If called with a 'layer' and a 'feature' parameter, the function will return the representation of the attributes of the specified feature from the specified layer.

语法	represent_attributes(layer, feature)
参数	layer - The layer (or its ID or name). feature - The feature which should be evaluated.
Examples	represent_attributes('atlas_layer', @atlas_feature) → The representation of the attributes for the specified feature from the specified layer.

Further reading: represent_value

13.2.22.12. represent_value

Returns the configured representation value for a field value. It depends on the configured widget type. Often, this is useful for 'Value Map' widgets.

语法	represent_value(value, [fieldName]) [] 标记可选参数
参数	value - The value which should be resolved. Most likely a field. fieldName - The field name for which the widget configuration should be loaded.
Examples	represent_value("field_with_value_map") → Description for value represent_value('static value', 'field_name') → Description for static value

Further reading: widget types, represent_attributes

13.2.22.13. sqlite_fetch_and_increment

Manage autoincrementing values in sqlite databases.

SQlite default values can only be applied on insert and not prefetched.

This makes it impossible to acquire an incremented primary key via AUTO_INCREMENT before creating the row in the database. Sidenote: with postgres, this works via the option evaluate default values.

When adding new features with relations, it is really nice to be able to already add children for a parent, while the parents form is still open and hence the parent feature uncommitted.

To get around this limitation, this function can be used to manage sequence values in a separate table on sqlite based formats like gpkg.

The sequence table will be filtered for a sequence id (filter_attribute and filter_value) and the current value of the id_field will be incremented by 1 and the incremented value returned.

If additional columns require values to be specified, the default_values map can be used for this purpose.

Note

This function modifies the target sqlite table. It is intended for usage with default value configurations for attributes.

When the database parameter is a layer and the layer is in transaction mode, the value will only be retrieved once during the lifetime of a transaction and cached and incremented. This makes it unsafe to work on the same database from several processes in parallel.

语法	sqlite_fetch_and_increment(database, table, id_field, filter_attribute, filter_value, [default_values]) [] 标记可选参数
参数	database - Path to the sqlite file or geopackage layer table - Name of the table that manages the sequences id_field - Name of the field that contains the current value filter_attribute - Name the field that contains a unique identifier for this sequence. Must have a UNIQUE index. filter_value - Name of the sequence to use. default_values - Map with default values for additional columns on the table. The values need to be fully quoted. Functions are allowed.
Examples	sqlite_fetch_and_increment(@layer, 'sequence_table', 'last_unique_id', 'sequence_id', 'global', map('last_change', 'date(''now'')', 'user', '''' || @user_account_name || '''')) → 0 sqlite_fetch_and_increment(layer_property(@layer, 'path'), 'sequence_table', 'last_unique_id', 'sequence_id', 'global', map('last_change', 'date(''now'')', 'user', '''' || @user_account_name || '''')) → 0

Further reading: 数据源属性, Creating one or many to many relations

13.2.22.14. uuid

Generates a Universally Unique Identifier (UUID) for each row using the Qt QUuid::createUuid method.

语法	uuid([format='WithBraces']) [] 标记可选参数
参数	format - The format, as the UUID will be formatted. 'WithBraces', 'WithoutBraces' or 'Id128'.
Examples	uuid() → '{0bd2f60f-f157-4a6d-96af-d4ba4cb366a1}' uuid('WithoutBraces') → '0bd2f60f-f157-4a6d-96af-d4ba4cb366a1' uuid('Id128') → '0bd2f60ff1574a6d96afd4ba4cb366a1'

13.2.23. Relations

This group contains the list of the relations available in the current project, with their description. It provides a quick access to the relation ID for writing an expression (with e.g. the relation_aggregate function) or customizing a form.

13.2.24. String Functions

This group contains functions that operate on strings (e.g., that replace, convert to upper case).

13.2.24.1. ascii

Returns the unicode code associated with the first character of a string.

语法	ascii(string)
参数	string - the string to convert to unicode code
Examples	ascii('Q') → 81

13.2.24.2. char

Returns the character associated with a unicode code.

语法	char(code)
参数	code - a unicode code number
Examples	char(81) → 'Q'

13.2.24.3. concat

Concatenates several strings to one. NULL values are converted to empty strings. Other values (like numbers) are converted to strings.

语法	concat(string1, string2, ...)
参数	string - a string value
Examples	concat('sun', 'set') → 'sunset' concat('a','b','c','d','e') → 'abcde' concat('Anno ', 1984) → 'Anno 1984' concat('The Wall', NULL) → 'The Wall'

About fields concatenation

You can also concatenate strings or field values using either || or + operators, with some special characteristics:

• The + operator also means sum up expression, so if you have an integer (field or numeric value) operand, this can be error prone and you better use the others:

  'My feature id is: ' + "gid" => triggers an error as gid returns an integer
  

• When any of the arguments is a NULL value, either || or + will return a NULL value. To return the other arguments regardless the NULL value, you may want to use the concat function:

  'My feature id is: ' + NULL ==> NULL
  'My feature id is: ' || NULL => NULL
  concat('My feature id is: ', NULL) => 'My feature id is: '
  

further reading: ||, +

13.2.24.4. format

Format a string using supplied arguments.

语法	format(string, arg1, arg2, ...)
参数	string - A string with placeholders %1, %2, etc., for the arguments. Placeholders can be repeated. The lowest numbered placeholder is replaced by arg1, the next by arg2, etc. arg - any type. Any number of arguments.
Examples	format('This %1 a %2','is', 'test') → 'This is a test' format('This is %2','a bit unexpected but 2 is lowest number in string','normal') → 'This is a bit unexpected but 2 is lowest number in string'

13.2.24.5. format_date

Formats a date type or string into a custom string format. Uses Qt date/time format strings. See QDateTime::toString.

语法	format_date(datetime, format, [language]) [] 标记可选参数
参数	datetime - date, time or datetime value format - String template used to format the string. Expression Output d the day as number without a leading zero (1 to 31) dd the day as number with a leading zero (01 to 31) ddd the abbreviated localized day name (e.g. 'Mon' to 'Sun') dddd the long localized day name (e.g. 'Monday' to 'Sunday') M the month as number without a leading zero (1-12) MM the month as number with a leading zero (01-12) MMM the abbreviated localized month name (e.g. 'Jan' to 'Dec') MMMM the long localized month name (e.g. 'January' to 'December') yy the year as two digit number (00-99) yyyy the year as four digit number These expressions may be used for the time part of the format string: Expression Output h the hour without a leading zero (0 to 23 or 1 to 12 if AM/PM display) hh the hour with a leading zero (00 to 23 or 01 to 12 if AM/PM display) H the hour without a leading zero (0 to 23, even with AM/PM display) HH the hour with a leading zero (00 to 23, even with AM/PM display) m the minute without a leading zero (0 to 59) mm the minute with a leading zero (00 to 59) s the second without a leading zero (0 to 59) ss the second with a leading zero (00 to 59) z the milliseconds without trailing zeroes (0 to 999) zzz the milliseconds with trailing zeroes (000 to 999) AP or A interpret as an AM/PM time. AP must be either 'AM' or 'PM'. ap or a Interpret as an AM/PM time. ap must be either 'am' or 'pm'. language - language (lowercase, two- or three-letter, ISO 639 language code) used to format the date into a custom string. By default the current QGIS user locale is used.
Examples	format_date('2012-05-15','dd.MM.yyyy') → '15.05.2012' format_date('2012-05-15','d MMMM yyyy','fr') → '15 mai 2012' format_date('2012-05-15','dddd') → 'Tuesday', if the current locale is an English variant format_date('2012-05-15 13:54:20','dd.MM.yy') → '15.05.12' format_date('13:54:20','hh:mm AP') → '01:54 PM'

13.2.24.6. format_number

Returns a number formatted with the locale separator for thousands. By default the current QGIS user locale is used. Also truncates the decimal places to the number of supplied places.

语法	format_number(number, [places=0], [language], [omit_group_separators=false], [trim_trailing_zeroes=false]) [] 标记可选参数
参数	number - number to be formatted places - integer representing the number of decimal places to truncate the string to. language - language (lowercase, two- or three-letter, ISO 639 language code) used to format the number into a string. By default the current QGIS user locale is used. omit_group_separators - if set to true then group separators will not be included in the string trim_trailing_zeroes - if set to true then trailing zeros following the decimal point will be trimmed from the string
Examples	format_number(10000000.332,2) → '10,000,000.33' if e.g. the current locale is an English variant format_number(10000000.332,2,'fr') → '10 000 000,33'

13.2.24.7. left

Returns a substring that contains the n leftmost characters of the string.

语法	left(string, length)
参数	string - a string length - integer. The number of characters from the left of the string to return.
Examples	left('Hello World',5) → 'Hello'

13.2.24.8. length

Returns the number of characters in a string or the length of a geometry linestring.

String variant

Returns the number of characters in a string.

语法	length(string)
参数	string - string to count length of
Examples	length('hello') → 5

Geometry variant

Calculate the length of a geometry line object. Calculations are always planimetric in the Spatial Reference System (SRS) of this geometry, and the units of the returned length will match the units for the SRS. This differs from the calculations performed by the $length function, which will perform ellipsoidal calculations based on the project's ellipsoid and distance unit settings.

语法	length(geometry)
参数	geometry - line geometry object
Examples	length(geom_from_wkt('LINESTRING(0 0, 4 0)')) → 4.0

13.2.24.9. lower

Converts a string to lower case letters.

语法	lower(string)
参数	string - the string to convert to lower case
Examples	lower('HELLO World') → 'hello world'

13.2.24.10. lpad

Returns a string padded on the left to the specified width, using a fill character. If the target width is smaller than the string's length, the string is truncated.

语法	lpad(string, width, fill)
参数	string - string to pad width - length of new string fill - character to pad the remaining space with
Examples	lpad('Hello', 10, 'x') → 'xxxxxHello' lpad('Hello', 3, 'x') → 'Hel'

13.2.24.11. regexp_match

Return the first matching position matching a regular expression within an unicode string, or 0 if the substring is not found.

语法	regexp_match(input_string, regex)
参数	input_string - the string to test against the regular expression regex - The regular expression to test against. Backslash characters must be double escaped (e.g., "\\s" to match a white space character or "\\b" to match a word boundary).
Examples	regexp_match('QGIS ROCKS','\\sROCKS') → 5 regexp_match('Budač','udač\\b') → 2

13.2.24.12. regexp_replace

Returns a string with the supplied regular expression replaced.

语法	regexp_replace(input_string, regex, replacement)
参数	input_string - the string to replace matches in regex - The regular expression to replace. Backslash characters must be double escaped (e.g., "\\s" to match a white space character). replacement - The string that will replace any matching occurrences of the supplied regular expression. Captured groups can be inserted into the replacement string using \\1, \\2, etc.
Examples	regexp_replace('QGIS SHOULD ROCK','\\sSHOULD\\s',' DOES ') → 'QGIS DOES ROCK' regexp_replace('ABC123','\\d+','') → 'ABC' regexp_replace('my name is John','(.*) is (.*)','\\2 is \\1') → 'John is my name'

13.2.24.13. regexp_substr

Returns the portion of a string which matches a supplied regular expression.

语法	regexp_substr(input_string, regex)
参数	input_string - the string to find matches in regex - The regular expression to match against. Backslash characters must be double escaped (e.g., "\\s" to match a white space character).
Examples	regexp_substr('abc123','(\\d+)') → '123'

13.2.24.14. replace

Returns a string with the supplied string, array, or map of strings replaced.

String & array variant

Returns a string with the supplied string or array of strings replaced by a string or an array of strings.

语法	replace(string, before, after)
参数	string - the input string before - the string or array of strings to replace after - the string or array of strings to use as a replacement
Examples	replace('QGIS SHOULD ROCK','SHOULD','DOES') → 'QGIS DOES ROCK' replace('QGIS ABC',array('A','B','C'),array('X','Y','Z')) → 'QGIS XYZ' replace('QGIS',array('Q','S'),'') → 'GI'

Map variant

Returns a string with the supplied map keys replaced by paired values. Longer map keys are evaluated first.

语法	replace(string, map)
参数	string - the input string map - the map containing keys and values
Examples	replace('APP SHOULD ROCK',map('APP','QGIS','SHOULD','DOES')) → 'QGIS DOES ROCK' replace('forty two',map('for','4','two','2','forty two','42')) → '42'

13.2.24.15. right

Returns a substring that contains the n rightmost characters of the string.

语法	right(string, length)
参数	string - a string length - integer. The number of characters from the right of the string to return.
Examples	right('Hello World',5) → 'World'

13.2.24.16. rpad

Returns a string padded on the right to the specified width, using a fill character. If the target width is smaller than the string's length, the string is truncated.

语法	rpad(string, width, fill)
参数	string - string to pad width - length of new string fill - character to pad the remaining space with
Examples	rpad('Hello', 10, 'x') → 'Helloxxxxx' rpad('Hello', 3, 'x') → 'Hel'

13.2.24.17. strpos

Return the first matching position of a substring within another string, or 0 if the substring is not found.

语法	strpos(haystack, needle)
参数	haystack - string that is to be searched needle - string to search for
Examples	strpos('HELLO WORLD','WORLD') → 7 strpos('HELLO WORLD','GOODBYE') → 0

13.2.24.18. substr

Returns a part of a string.

语法	substr(string, start, [length]) [] 标记可选参数
参数	string - the full input string start - integer representing start position to extract beginning with 1; if start is negative, the return string will begin at the end of the string minus the start value length - integer representing length of string to extract; if length is negative, the return string will omit the given length of characters from the end of the string
Examples	substr('HELLO WORLD',3,5) → 'LLO W' substr('HELLO WORLD',6) → ' WORLD' substr('HELLO WORLD',-5) → 'WORLD' substr('HELLO',3,-1) → 'LL' substr('HELLO WORLD',-5,2) → 'WO' substr('HELLO WORLD',-5,-1) → 'WORL'

13.2.24.19. title

Converts all words of a string to title case (all words lower case with leading capital letter).

语法	title(string)
参数	string - the string to convert to title case
Examples	title('hello WOrld') → 'Hello World'

13.2.24.20. to_string

Converts a number to string.

语法	to_string(number)
参数	number - Integer or real value. The number to convert to string.
Examples	to_string(123) → '123'

13.2.24.21. trim

Removes all leading and trailing whitespace (spaces, tabs, etc) from a string.

语法	trim(string)
参数	string - string to trim
Examples	trim('   hello world  ') → 'hello world'

13.2.24.22. upper

Converts a string to upper case letters.

语法	upper(string)
参数	string - the string to convert to upper case
Examples	upper('hello WOrld') → 'HELLO WORLD'

13.2.24.23. wordwrap

Returns a string wrapped to a maximum/minimum number of characters.

语法	wordwrap(string, wrap_length, [delimiter_string]) [] 标记可选参数
参数	string - the string to be wrapped wrap_length - an integer. If wrap_length is positive the number represents the ideal maximum number of characters to wrap; if negative, the number represents the minimum number of characters to wrap. delimiter_string - Optional delimiter string to wrap to a new line.
Examples	wordwrap('UNIVERSITY OF QGIS',13) → 'UNIVERSITY OF<br>QGIS' wordwrap('UNIVERSITY OF QGIS',-3) → 'UNIVERSITY<br>OF QGIS'

13.2.25. User Expressions

This group contains the expressions saved as user expressions.

13.2.26. Variables

This group contains dynamic variables related to the application, the project file and other settings. The availability of variables depends on the context:

• from the ^{Select by expression} dialog

• from the ^{Field calculator} dialog

• from the layer properties dialog

• from the print layout

To use these variables in an expression, they should be preceded by the @ character (e.g, @row_number).

Variable	描述
algorithm_id	The unique ID of an algorithm
animation_end_time	End of the animation's overall temporal time range (as a datetime value)
animation_interval	Duration of the animation's overall temporal time range (as an interval value)
animation_start_time	Start of the animation's overall temporal time range (as a datetime value)
atlas_feature	The current atlas feature (as feature object)
atlas_featureid	The current atlas feature ID
atlas_featurenumber	The current atlas feature number in the layout
atlas_filename	The current atlas file name
atlas_geometry	The current atlas feature geometry
atlas_layerid	The current atlas coverage layer ID
atlas_layername	The current atlas coverage layer name
atlas_pagename	The current atlas page name
atlas_totalfeatures	The total number of features in atlas
canvas_cursor_point	The last cursor position on the canvas in the project's geographical coordinates
cluster_color	The color of symbols within a cluster, or NULL if symbols have mixed colors
cluster_size	The number of symbols contained within a cluster
current_feature	The feature currently being edited in the attribute form or table row
current_geometry	The geometry of the feature currently being edited in the form or the table row
current_parent_feature	represents the feature currently being edited in the parent form. Only usable in an embedded form context.
current_parent_geometry	represents the geometry of the feature currently being edited in the parent form. Only usable in an embedded form context.
form_mode	What the form is used for, like AddFeatureMode, SingleEditMode, MultiEditMode, SearchMode, AggregateSearchMode or IdentifyMode as string.
feature NEW in 3.28	The current feature being evaluated. This can be used with the 'attribute' function to evaluate attribute values from the current feature.
frame_duration	Temporal duration of each animation frame (as an interval value)
frame_number	Current frame number during animation playback
frame_rate	Number of frames per second during animation playback
fullextent_maxx	Maximum x value from full canvas extent (including all layers)
fullextent_maxy	Maximum y value from full canvas extent (including all layers)
fullextent_minx	Minimum x value from full canvas extent (including all layers)
fullextent_miny	Minimum y value from full canvas extent (including all layers)
geometry NEW in 3.28	The geometry of the current feature being evaluated
geometry_part_count	The number of parts in rendered feature's geometry
geometry_part_num	The current geometry part number for feature being rendered
geometry_point_count	The number of points in the rendered geometry's part
geometry_point_num	The current point number in the rendered geometry's part
geometry_ring_num	Current geometry ring number for feature being rendered (for polygon features only). The exterior ring has a value of 0.
grid_axis	The current grid annotation axis (eg, 'x' for longitude, 'y' for latitude)
grid_number	The current grid annotation value
id NEW in 3.28	The ID of the current feature being evaluated
item_id	The layout item user ID (not necessarily unique)
item_uuid	The layout item unique ID
layer	The current layer
layer_crs	The Coordinate Reference System Authority ID of the current layer
layer_id	The ID of current layer
layer_ids	The IDs of all the map layers in the current project as a list
layer_name	The name of current layer
layers	All the map layers in the current project as a list
layout_dpi	The composition resolution (DPI)
layout_name	The layout name
layout_numpages	The number of pages in the layout
layout_page	The page number of the current item in the layout
layout_pageheight	The active page height in the layout (in mm for standard paper sizes, or whatever unit was used for custom paper size)
layout_pageoffsets	Array of Y coordinate of the top of each page. Allows to dynamically position items on pages in a context where page sizes may change
layout_pagewidth	The active page width in the layout (in mm for standard paper sizes, or whatever unit was used for custom paper size)
legend_column_count	The number of columns in the legend
legend_filter_by_map	Indicates if the content of the legend is filtered by the map
legend_filter_out_atlas	Indicates if the atlas is filtered out of the legend
legend_split_layers	Indicates if layers can be split in the legend
legend_title	The title of the legend
legend_wrap_string	The character(s) used to wrap the legend text
map_crs	The Coordinate reference system of the current map
map_crs_acronym	The acronym of the Coordinate reference system of the current map
map_crs_definition	The full definition of the Coordinate reference system of the current map
map_crs_description	The name of the Coordinate reference system of the current map
map_crs_ellipsoid	The acronym of the ellipsoid of the Coordinate reference system of the current map
map_crs_proj4	The Proj4 definition of the Coordinate reference system of the current map
map_crs_projection	The descriptive name of the projection method used by the Coordinate reference system of the map (e.g. 'Albers Equal Area')
map_crs_wkt	The WKT definition of the Coordinate reference system of the current map
map_end_time	The end of the map's temporal time range (as a datetime value)
map_extent	The geometry representing the current extent of the map
map_extent_center	The point feature at the center of the map
map_extent_height	The current height of the map
map_extent_width	The current width of the map
map_id	The ID of current map destination. This will be 'canvas' for canvas renders, and the item ID for layout map renders
map_interval	The duration of the map's temporal time range (as an interval value)
map_layer_ids	The list of map layer IDs visible in the map
map_layers	The list of map layers visible in the map
map_rotation	The current rotation of the map
map_scale	The current scale of the map
map_start_time	The start of the map's temporal time range (as a datetime value)
map_units	The units of map measurements
model_path	Full path (including file name) of current model (or project path if model is embedded in a project).
model_folder	Folder containing current model (or project folder if model is embedded in a project).
model_name	Name of current model
model_group	Group for current model
notification_message	Content of the notification message sent by the provider (available only for actions triggered by provider notifications).
parent	Refers to the current feature in the parent layer, providing access to its attributes and geometry when filtering an aggregate function
project_abstract	The project abstract, taken from project metadata
project_area_units	The area unit for the current project, used when calculating areas of geometries
project_author	The project author, taken from project metadata
project_basename	The basename of current project's filename (without path and extension)
project_creation_date	The project creation date, taken from project metadata
project_crs	The Coordinate reference system of the project
project_crs_arconym	The acronym of the Coordinate reference system of the project
project_crs_definition	The full definition of the Coordinate reference system of the project
project_crs_description	The description of the Coordinate reference system of the project
project_crs_ellipsoid	The ellipsoid of the Coordinate reference system of the project
project_crs_proj4	The Proj4 representation of the Coordinate reference system of the project
project_crs_wkt	The WKT (well known text) representation of the coordinate reference system of the project
project_distance_units	The distance unit for the current project, used when calculating lengths of geometries and distances
project_ellipsoid	The name of the ellipsoid of the current project, used when calculating geodetic areas or lengths of geometries
project_filename	The filename of the current project
project_folder	The folder of the current project
project_home	The home path of the current project
project_identifier	The project identifier, taken from the project's metadata
project_keywords	The project keywords, taken from the project's metadata
project_last_saved	Date/time when project was last saved.
project_path	The full path (including file name) of the current project
project_title	The title of current project
project_units	The units of the project's CRS
qgis_locale	The current language of QGIS
qgis_os_name	The current Operating system name, eg 'windows', 'linux' or 'osx'
qgis_platform	The QGIS platform, eg 'desktop' or 'server'
qgis_release_name	The current QGIS release name
qgis_short_version	The current QGIS version short string
qgis_version	The current QGIS version string
qgis_version_no	The current QGIS version number
row_number	Stores the number of the current row
snapping_results	Gives access to snapping results while digitizing a feature (only available in add feature)
scale_value	The current scale bar distance value
selected_file_path	Selected file path from file widget selector when uploading a file with an external storage system
symbol_angle	The angle of the symbol used to render the feature (valid for marker symbols only)
symbol_color	The color of the symbol used to render the feature
symbol_count	The number of features represented by the symbol (in the layout legend)
symbol_id	The Internal ID of the symbol (in the layout legend)
symbol_label	The label for the symbol (either a user defined label or the default autogenerated label - in the layout legend)
symbol_layer_count	Total number of symbol layers in the symbol
symbol_layer_index	Current symbol layer index
symbol_marker_column	Column number for marker (valid for point pattern fills only).
symbol_marker_row	Row number for marker (valid for point pattern fills only).
user_account_name	The current user's operating system account name
user_full_name	The current user's operating system user name
value	The current value
vector_tile_zoom	Exact vector tile zoom level of the map that is being rendered (derived from the current map scale). Normally in interval [0, 20]. Unlike @zoom_level, this variable is a floating point value which can be used to interpolate values between two integer zoom levels.
with_variable	Allows setting a variable for usage within an expression and avoid recalculating the same value repeatedly
zoom_level	Vector tile zoom level of the map that is being rendered (derived from the current map scale). Normally in interval [0, 20].

Some examples:

• Return the X coordinate of a map item center in layout:

  x( map_get( item_variables( 'map1'), 'map_extent_center' ) )
  

• Return, for each feature in the current layer, the number of overlapping airport features:

  aggregate( layer:='airport', aggregate:='count', expression:="code",
                 filter:=intersects( $geometry, geometry( @parent ) ) )
  

• Get the object_id of the first snapped point of a line:

  with_variable(
    'first_snapped_point',
    array_first( @snapping_results ),
    attribute(
      get_feature_by_id(
        map_get( @first_snapped_point, 'layer' ),
        map_get( @first_snapped_point, 'feature_id' )
      ),
      'object_id'
    )
  )
  

13.2.27. Recent Functions

This group contains recently used functions. Depending on the context of its usage (feature selection, field calculator, generic), recently applied expressions are added to the corresponding list (up to ten expressions), sorted from more to less recent. This makes it easy to quickly retrieve and reapply previously used expressions.