SQL OUTER JOIN

SQL > SQL Advanced > SQL OUTER JOIN

What is SQL OUTER JOIN?

The SQL OUTER JOIN clause is a variation of the SQL JOIN clause enables a SELECT statement to access more than one table. The JOIN clause controls how tables are linked. It is a qualifier of the SQL FROM clause.

The OUTER JOIN clause differs from the standard JOIN clause (also known as the INNER JOIN clause) in that rows are returned even when there are no matches through the JOIN critieria on the second table.

Why Use SQL OUTER JOIN?

Use the SQL OUTER JOIN whenever multiple tables must be accessed through a SQL SELECT statement and results should be returned if there is not a match between the JOINed tables.
It can be useful when there is a need to merge data from two tables and to include all rows from both tables without depending on a match. Another use is to generate a large result set for testing purposes.

How To Use SQL OUTER JOIN

SQL OUTER JOIN is used as follows. The ON clause describes the conditions of the JOIN.

Important! A "cartesian product" can result if there is no relating the tables for the join. A row would be included for each combination between the two tables so if one table has 1,000 rows and the second table has 2,000 rows then 2,000,000 rows would be returned.

Important! If there are matches on the JOIN criteria then rows will still be returned. This is known an "OUTER JOIN". Use the "INNER JOIN" in cases where no rows should be returned when one side of the join is missing.

SQL OUTER JOIN Syntax

SELECT <column_name1>, <column_name2> <aggregate_function>
FROM <table_name>
LEFT OUTER JOIN <table_name> ON <join_conditions>

SQL OUTER JOIN Example

The following example JOINs the region and branch tables on the region_nbr column.

Here are the contents of the tables:

Table: REGION

region_nbr region_name

100 East Region

200 Central Region

300 Virtual Region

400 West Region

Table: BRANCH

branch_nbr branch_name region_nbr employee_count

108 New York 100 10

110 Boston 100 6

212 Chicago 200 5

404 San Diego 400 6

415 San Jose 400 3

This SQL Statement with OUTER JOIN is executed:

SELECT region.region_nbr, region.region_name, branch.branch_nbr, branch.branch_name
FROM dbo.region
LEFT OUTER JOIN dbo.branch
ON branch.region_nbr = region.region_nbr
ORDER BY region.region_nbr

Here is the result. Note that the "Virtual Region" is included in the results even though it has no rows in the branch table. This is the difference between the INNER JOIN and OUTER JOIN.

region_nbr region_name branch_nbr branch_name

100 East Region 108 New York

100 East Region 110 Boston

200 Central Region 212 Chicago

300 Virtual Region NULL NULL

400 West Region 404 San Diego

400 West Region 415 San Jose