Purpose

You can use a Join as a way to associate multiple tables within a Select statement. Joins may be either explicit or implicit. For example, the following is the example from the previous section restated as an explicit inner join:

SELECT * FROM emp INNER JOIN dep ON id=empId
SELECT e.name, d.deptName 
FROM emp e INNER JOIN dep d ON e.deptId = d.id;

whereas the following is the same statement as an implicit inner join:

SELECT * FROM emp, dep WHERE emp.deptID=dep.id
Note: The ON clause in a join expression must evaluate to a true or false value.

Syntax

FROM table_name {RIGHT OUTER | INNER | LEFT OUTER | CROSS | FULL OUTER} JOIN table.key ON search-condition

Example

In this example, two tables are joined using LEFT OUTER JOIN. T1, the first table named includes nonmatching rows.

SELECT * FROM T1 LEFT OUTER JOIN T2 ON T1.key = T2.key

If you use a CROSS JOIN, no ON expression is allowed for the join.