What is SQL JOIN?
The SQL JOIN is a clause that 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 standard JOIN clause (also known as the INNER JOIN clause) differs from the OUTER JOIN in that rows are returned only when there are matches for the JOIN critieria on the second table.
Why Use SQL JOIN?
Use the SQL JOIN whenever multiple tables must be accessed through a SQL SELECT statement and no results should be returned if there is not a match between the JOINed tables.
How To Use 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
SQL JOIN Example
The following example JOINs the region and branch tables on the region_nbr column.
Here are the contents of the tables:
This SQL Statement with JOIN is executed:
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.