Difference between revisions of "Oracle:Joining tables"
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!colspan="3"|DEPT | !colspan="3"|DEPT | ||
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| − | !EMPNO!!ENAME!!JOB!!.....!!DEPTNO!! !!DEPTNO!!DNAME!!...... | + | !EMPNO!!ENAME!!JOB!!.....!!DEPTNO!! !!DEPTNO!!DNAME!!...... |
|- | |- | ||
| − | |7499||ALLEN||SALES||.....||30|| & | + | |7499||ALLEN||SALES||.....||30|| ||30||SALES||...... |
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!colspan="4" | | !colspan="4" | | ||
Revision as of 11:24, 4 March 2016
Main Page >> Oracle and SQL >> Workbook >> Joining tables
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
Next Step
Return to the Workbook.
