15.3. Lesson: Adding Data to the Model

The models we’ve created will now need to be populated with the data they’re intended to contain.

The goal for this lesson: To learn how to insert new data into the database models.

15.3.1. Insert statement

How do you add data to a table? The sql INSERT statement provides the functionality for this:

insert into streets (name) values ('High street');

A couple of things to note:

  • After the table name (streets), you list the column names that you will be populating (in this case only the name column).

  • After the values keyword, place the list of field values.

  • Strings should be quoted using single quotes.

  • Note that we did not insert a value for the id column; this is because it is a sequence and will be auto-generated.

  • If you do manually set the id, you may cause serious problems with the integrity of your database.

You should see INSERT 0 1 if it is successful.

You can see the result of your insert action by selecting all the data in the table:

select * from streets;

Result:

select * from streets;
 id |    name
----+-------------
  1 | High street
(1 row)

Try Yourself basic

Use the INSERT command to add a new street to the streets table.

Answer

The SQL command you should use looks like this (you can replace the street name with a name of your choice):

insert into streets (name) values ('Low Road');

15.3.2. Sequencing Data Addition According to Constraints

15.3.3. Try Yourself moderate

Try to add a person object to the people table with the following details:

Name: Joe Smith
House Number: 55
Street: Main Street
Phone: 072 882 33 21

Note

Recall that in this example, we defined phone numbers as strings, not integers.

At this point, you should have an error report if you try to do this without first creating a record for Main Street in the streets table.

You should have also noticed that:

  • You can’t add the street using its name

  • You can’t add a street using a street id before first creating the street record on the streets table

Remember that our two tables are linked via a Primary/Foreign Key pair. This means that no valid person can be created without there also being a valid corresponding street record.

Using the above knowledge, add the new person to the database.

Answer

Here is the correct SQL statement:

insert into streets (name) values('Main Road');
insert into people (name,house_no, street_id, phone_no)
  values ('Joe Smith',55,2,'072 882 33 21');

If you look at the streets table again (using a select statement as before), you’ll see that the id for the Main Road entry is 2.

That’s why we could merely enter the number 2 above. Even though we’re not seeing Main Road written out fully in the entry above, the database will be able to associate that with the street_id value of 2.

If you have already added a new street object, you might find that the new Main Road has an id of 3 not 2.

15.3.4. Select data

We have already shown you the syntax for selecting records. Let’s look at a few more examples:

select name from streets;

select * from streets;

select * from streets where name='Main Road';

In later sessions we will go into more detail on how to select and filter data.

15.3.5. Update data

What if you want to make a change to some existing data? For example, a street name is changed:

update streets set name='New Main Road' where name='Main Road';

Be very careful using such update statements - if more than one record matches your WHERE clause, they will all be updated!

A better solution is to use the primary key of the table to reference the record to be changed:

update streets set name='New Main Road' where id=2;

It should return UPDATE 1.

Note

the WHERE statement criteria are case sensitive Main Road is not the same as Main road

15.3.6. Delete Data

In order to delete an object from a table, use the DELETE command:

delete from people where name = 'Joe Smith';

Let’s look at our people table now:

address=# select * from people;

  id | name | house_no | street_id | phone_no
 ----+------+----------+-----------+----------
(0 rows)

15.3.7. Try Yourself hard

Use the skills you have learned to add some new friends to your database:

      name       | house_no | street_id |   phone_no
-----------------+----------+-----------+--------------
Joe Bloggs       |        3 |         2 | 072 887 23 45
Jane Smith       |       55 |         3 | 072 837 33 35
Roger Jones      |       33 |         1 | 072 832 31 38
Sally Norman     |       83 |         1 | 072 932 31 32

15.3.8. In Conclusion

Now you know how to add new data to the existing models you created previously. Remember that if you want to add new kinds of data, you may want to modify and/or create new models to contain that data.

15.3.9. What’s Next?

Now that you’ve added some data, you’ll learn how to use queries to access this data in various ways.