Oracle:Joining tables

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Joining Tables Introduction

Sometimes information needs to be retrieved from more than one table. The relationships between rows in one table and rows in another are established by the values in certain corresponding columns (foreign key).

For example:

EMP						DEPT
EMPNO	ENAME	JOB	.....	DEPTNO		DEPTNO	DNAME	......
7499	ALLEN	SALES	.....	30		30	SALES	......
				↑ join attributes ↑

The table must have matching values in the join attributes to enable a join to take place. If there are no matching values, the tables will not join! For example, if 30 was missing from the DEPT table, then the employees from department 30 would not appear in any output that joins tables DEPT and EMP together. (A consequence of violating referential integrity!)

There are different ways to join tables:

• Basic Join conditions
• Recursive joins
• Outer joins

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