15.5. Lesson: Views¶
When you write a query, you need to spend a lot of time and effort formulating it. With views, you can save the definition of an SQL query in a reusable “virtual table”.
The goal for this lesson: To save a query as a view.
15.5.1. Creare una Vista¶
Puoi considerare una vista proprio come un tabella, ma i suoi dati sono recuperati da una query. Cerchiamo di fare una una semplice vista basata sull’esempio soprastante:
create view roads_count_v as select count(people.name), streets.name from people, streets where people.street_id=streets.id group by people.street_id, streets.name;
As you can see the only change is the create view roads_count_v as part at the beginning. We can now select data from that view:
select * from roads_count_v;
count | name -------+------------- 1 | Main Road 2 | High street 1 | Low Street (3 rows)
15.5.2. Modifying a View¶
A view is not fixed, and it contains no “real data”. This means you can easily change it without impacting on any data in your database:
CREATE OR REPLACE VIEW roads_count_v AS SELECT count(people.name), streets.name FROM people, streets WHERE people.street_id=streets.id GROUP BY people.street_id, streets.name ORDER BY streets.name;
(This example also shows the best practice convention of using UPPER CASE for all SQL keywords.)
You will see that we have added an ORDER BY clause so that our view rows are nicely sorted:
select * from roads_count_v; count | name -------+------------- 2 | High street 1 | Low Street 1 | Main Road (3 rows)
15.5.3. Dropping a View¶
If you no longer need a view, you can delete it like this:
drop view roads_count_v;
15.5.4. In Conclusion¶
Using views, you can save a query and access its results as if it were a table.
15.5.5. What’s Next?¶
Sometimes, when changing data, you want your changes to have effects elsewhere in the database. The next lesson will show you how to do this.