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SQL Lesson 7: OUTER JOINs
Depending on how you want to analyze the data, the INNER JOIN we used last lesson might not be
sufficient because the resulting table only contains data that belongs in both of the tables.
If the two tables have asymmetric data, which can easily happen when data is entered in different
stages, then we would have to use a LEFT JOIN, RIGHT JOIN or FULL JOIN instead to ensure that
the data you need is not left out of the results.
Select query with LEFT/RIGHT/FULL JOINs on multiple tables
SELECT column, another_column, …
FROM mytable
INNER/LEFT/RIGHT/FULL JOIN another_table
ON mytable.id = another_table.matching_id
WHERE condition(s)
ORDER BY column, … ASC/DESC
LIMIT num_limit OFFSET num_offset;
Like the INNER JOIN these three new joins have to specify which column to join the data on.
When joining table A to table B, a LEFT JOIN simply includes rows from A regardless of whether
a matching row is found in B. The RIGHT JOIN is the same, but reversed, keeping rows in B
regardless of whether a match is found in A. Finally, a FULL JOIN simply means that rows from
both tables are kept, regardless of whether a matching row exists in the other table.
When using any of these new joins, you will likely have to write additional logic to deal with NULLs
in the result and constraints (more on this in the next lesson).
Did you know?
You might see queries with these joins written as LEFT OUTER JOIN,
RIGHT OUTER JOIN, or FULL OUTER JOIN, but the OUTER
keyword is really kept for SQL-92 compatibility and these queries are simply equivalent to
LEFT JOIN, RIGHT JOIN, and FULL JOIN respectively.
Exercise
In this exercise, you are going to be working with a new table which stores fictional data about Employees in the film studio and their assigned office Buildings. Some of the buildings are new, so they don't have any employees in them yet, but we need to find some information about them regardless.
Since our browser SQL database is somewhat limited, only the LEFT JOIN is supported in the
exercise below.
Table: buildings (Read-only)
Table: employees (Read-only)
Query results
Exercise 7 — Tasks
- Find the list of all buildings that have employees
- Find the list of all buildings and their capacity
- List all buildings and the distinct employee roles in each building (including empty buildings)
Stuck? Read this task's Solution.
Solve all tasks to continue to the next lesson.
Finish above Tasks
Solve all tasks to continue to the next lesson.