SQL JOIN

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

SQL JOIN Syntax

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

SQL 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 JOIN is executed:

SELECT region.region_nbr, region.region_name, branch.branch_nbr, branch.branch_name FROM dbo.region 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
400	West Region	404	San Diego
400	West Region	415	San Jose