Docs in progress for ‘QGIS testing’. Visit http://docs.qgis.org/2.18 for QGIS 2.18 docs and translations.

16.2. 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.

16.2.1. Install PostgreSQL

Note

Although outside the scope of this document, Mac users can install PostgreSQL using Homebrew. Windows users can use the graphical installer located here: http://www.postgresql.org/download/windows/. Please note that the documentation will assume users are running QGIS under Ubuntu.

Under Ubuntu:

sudo apt-get 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.

16.2.2. Help

PostgreSQL has very good online documentation.

16.2.3. 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

16.2.4. 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.

16.2.5. 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.

16.2.6. 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 - available online here.

16.2.7. 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.

16.2.7.1. Try Yourself moderate

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.

Check your results

16.2.8. 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.

../../../_images/er-people-streets.png

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)

16.2.9. 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)

16.2.10. Dropping Tables in SQL

If you want to get rid of a table you can use the drop command:

drop table streets;

Note

In our current example, the above command would not work. Why not? See why

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.

16.2.11. 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-get install pgadmin3

pgAdmin III will be covered in more detail in another module.

16.2.12. In Conclusion

You have now seen how to create a brand new database, starting completely from scratch.

16.2.13. What’s Next?

Next you’ll learn how to use the DBMS to add new data.