Joining Tables

Right now, we have seen querying being performed on only one table. If any information required to be queried from more than one table, a concept known as JOINS is used. This permits data to be selected from one or more tables and to combine the selected data into a single result table.

Cartesian Products

If a join condition is not specified, Oracle performs a Cartesian product. The Oracle combines every row of one table with each row of the other. For instance, if emp table contains 10 records and dept table contains 4 record and the number of rows selected through the query without a join condition yields 40 records. Constantly when data is selected from different tables, a join condition is needed.

Join Conditions

Most Join queries hold WHERE clause conditions which compare two columns, one from each table. Like a condition is known as a JOIN Condition. For executing a Join, Oracle combines pairs of rows and each containing one row from each table, for that the join condition evaluates to TRUE. Joins can be of different kinds:

•    Equi-Join
•    Self Join
•    Outer Join
•    Non-Equi Join