15.2. Lesson: Implementing the Data Model
Now that we’ve covered all the theory, let’s create a new database. This database will be used for our exercises for the lessons that will follow afterwards.
The goal for this lesson: To install the required software and use it to implement our example database.
15.2.1. Install PostgreSQL
Note
You can find PostgreSQL packages and installation instructions for your operating system at https://www.postgresql.org/download/. Please note that the documentation will assume users are running QGIS under Ubuntu.
Under Ubuntu:
sudo apt install postgresql-9.1
You should get a message like this:
[sudo] password for qgis:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
postgresql-client-9.1 postgresql-client-common postgresql-common
Suggested packages:
oidentd ident-server postgresql-doc-9.1
The following NEW packages will be installed:
postgresql-9.1 postgresql-client-9.1 postgresql-client-common postgresql-common
0 upgraded, 4 newly installed, 0 to remove and 5 not upgraded.
Need to get 5,012kB of archives.
After this operation, 19.0MB of additional disk space will be used.
Do you want to continue [Y/n]?
Press Y and Enter and wait for the download and installation to finish.
15.2.2. Help
PostgreSQL has very good online documentation.
15.2.3. Create a database user
Under Ubuntu:
After the installation is complete, run this command to become the postgres user and then create a new database user:
sudo su - postgres
Type in your normal log in password when prompted (you need to have sudo rights).
Now, at the postgres user’s bash prompt, create the database user. Make sure the user name matches your unix login name: it will make your life much easier, as postgres will automatically authenticate you when you are logged in as that user:
createuser -d -E -i -l -P -r -s qgis
Enter a password when prompted. You should use a different password to your login password.
What do those options mean?
-d, --createdb role can create new databases
-E, --encrypted encrypt stored password
-i, --inherit role inherits privileges of roles it is a member of (default)
-l, --login role can login (default)
-P, --pwprompt assign a password to new role
-r, --createrole role can create new roles
-s, --superuser role will be superuser
Now you should leave the postgres user’s bash shell environment by typing:
exit
15.2.4. Verify the new account
psql -l
Should return something like this:
Name | Owner | Encoding | Collation | Ctype |
----------+----------+----------+------------+------------+
postgres | postgres | UTF8 | en_ZA.utf8 | en_ZA.utf8 |
template0 | postgres | UTF8 | en_ZA.utf8 | en_ZA.utf8 |
template1 | postgres | UTF8 | en_ZA.utf8 | en_ZA.utf8 |
(3 rows)
Type Q to exit.
15.2.5. Create a database
The createdb command is used to create a new database. It should be run
from the bash shell prompt:
createdb address -O qgis
You can verify the existence of your new database by using this command:
psql -l
Which should return something like this:
Name | Owner | Encoding | Collation | Ctype | Access privileges
----------+----------+----------+------------+------------+-----------------------
address | qgis | UTF8 | en_ZA.utf8 | en_ZA.utf8 |
postgres | postgres | UTF8 | en_ZA.utf8 | en_ZA.utf8 |
template0 | postgres | UTF8 | en_ZA.utf8 | en_ZA.utf8 | =c/postgres: postgres=CTc/postgres
template1 | postgres | UTF8 | en_ZA.utf8 | en_ZA.utf8 | =c/postgres: postgres=CTc/postgres
(4 rows)
Type Q to exit.
15.2.6. Starting a database shell session
You can connect to your database easily like this:
psql address
To exit out of the psql database shell, type:
\q
For help in using the shell, type:
\?
For help in using sql commands, type:
\help
To get help on a specific command, type (for example):
\help create table
See also the Psql cheat sheet.
15.2.7. Make Tables in SQL
Let’s start making some tables! We will use our ER Diagram as a guide. First, connect to the address db:
psql address
Then create a streets table:
create table streets (id serial not null primary key, name varchar(50));
serial and varchar are data types. serial tells PostgreSQL to start an integer sequence (auto-number) to populate the id automatically for every new record. varchar(50) tells PostgreSQL to create a character field of 50 characters in length.
You will notice that the command ends with a ; - all SQL commands should be terminated this way. When you press Enter, psql will report something like this:
NOTICE: CREATE TABLE will create implicit sequence "streets_id_seq"
for serial column "streets.id"
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
"streets_pkey" for table "streets"
CREATE TABLE
That means your table was created successfully, with a primary key streets_pkey using streets.id.
Note: If you hit return without entering a ;, then you will get a prompt like this: address-#. This is because PG is expecting you to enter more. Enter ; to run your command.
To view your table schema, you can do this:
\d streets
Which should show something like this:
Table "public.streets"
Column | Type | Modifiers
--------+-----------------------+--------------------------------------
id | integer | not null default
| | nextval('streets_id_seq'::regclass)
name | character varying(50) |
Indexes:
"streets_pkey" PRIMARY KEY, btree (id)
To view your table contents, you can do this:
select * from streets;
Which should show something like this:
id | name
---+------
(0 rows)
As you can see, our table is currently empty.
Try Yourself: ★★☆
Use the approach shown above to make a table called people:
Add fields such as phone number, home address, name, etc. (these aren’t all valid names: change them to make them valid). Make sure you give the table an ID column with the same data-type as above.
Answer
The SQL required to create the correct people table is:
create table people (id serial not null primary key,
name varchar(50),
house_no int not null,
street_id int not null,
phone_no varchar null );
The schema for the table (enter \\d people) looks like this:
Table "public.people"
Column | Type | Modifiers
-----------+-----------------------+-------------------------------------
id | integer | not null default
| | nextval('people_id_seq'::regclass)
name | character varying(50) |
house_no | integer | not null
street_id | integer | not null
phone_no | character varying |
Indexes:
"people_pkey" PRIMARY KEY, btree (id)
For illustration purposes, we have purposely omitted the fkey constraint.
15.2.8. Create Keys in SQL
The problem with our solution above is that the database doesn’t know that people and streets have a logical relationship. To express this relationship, we have to define a foreign key that points to the primary key of the streets table.
There are two ways to do this:
Add the key after the table has been created
Define the key at time of table creation
Our table has already been created, so let’s do it the first way:
alter table people
add constraint people_streets_fk foreign key (street_id) references streets(id);
That tells the people table that its street_id fields must match a valid street id from the streets table.
The more usual way to create a constraint is to do it when you create the table:
create table people (id serial not null primary key,
name varchar(50),
house_no int not null,
street_id int references streets(id) not null,
phone_no varchar null);
\d people
After adding the constraint, our table schema looks like this now:
Table "public.people"
Column | Type | Modifiers
-----------+-----------------------+---------------------------------
id | integer | not null default
| | nextval('people_id_seq'::regclass)
name | character varying(50) |
house_no | integer | not null
street_id | integer | not null
phone_no | character varying |
Indexes:
"people_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
"people_streets_fk" FOREIGN KEY (id) REFERENCES streets(id)
15.2.9. Create Indexes in SQL
We want lightning fast searches on peoples names. To provide for this, we can create an index on the name column of our people table:
create index people_name_idx on people(name);
\d people
Which results in:
Table "public.people"
Column | Type | Modifiers
-----------+-----------------------+-----------------------------------
id | integer | not null default nextval
| | ('people_id_seq'::regclass)
name | character varying(50) |
house_no | integer | not null
street_id | integer | not null
phone_no | character varying |
Indexes:
"people_pkey" PRIMARY KEY, btree (id)
"people_name_idx" btree (name) <-- new index added!
Foreign-key constraints:
"people_streets_fk" FOREIGN KEY (id) REFERENCES streets(id)
15.2.10. Dropping Tables in SQL
If you want to get rid of a table you can use the drop command:
drop table streets;
In our current example, the above command would not work. Why not?
Answer
The reason the DROP command would not work in this case is because the people table has a Foreign Key constraint to the streets table. This means that dropping (or deleting) the streets table would leave the people table with references to non-existent streets data.
It is possible to ‘force’ the streets table to be deleted by using the
CASCADE command, but this would also delete the people and any other table
which had a relationship to the streets table. Use with caution!
If you used the same drop table command on the people table, it would be successful:
drop table people;
Note
If you actually did enter that command and dropped the people table, now would be a good time to rebuild it, as you will need it in the next exercises.
15.2.11. A word on pgAdmin III
We are showing you the SQL commands from the psql prompt because it’s a very useful way to learn about databases. However, there are quicker and easier ways to do a lot of what we are showing you. Install pgAdmin III and you can create, drop, alter etc tables using ‘point and click’ operations in a GUI.
Under Ubuntu, you can install it like this:
sudo apt install pgadmin3
pgAdmin III will be covered in more detail in another module.
15.2.12. In Conclusion
You have now seen how to create a brand new database, starting completely from scratch.
15.2.13. What’s Next?
Next you’ll learn how to use the DBMS to add new data.