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.

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