Objectivesj i

After completing this lesson, you should be able to do the following:

•• Writerite SELECT statementst t ts toto access datata fromfr morere thant one tablet le usingi equalitylity and nonequalitylity joinsj i

•• Viewi datata thatt t generallyr lly does nott meett a joinj in conditioniti by usingi outert r joinsj i

•• Joinin a tablet le toto itselfit lf

4-2

Obtainingi i Data from Multiplel i le Tablesl

EMPNO DEPTNO LOC

----- ------- --------

14 rows selected.

4-3

What IsIs a Join?i

Use a join to query data from more than one table.

SELECT table1.column, table2.column FROM table1, table2

WHERE table1.column1 = table2.column2;

•• Writerite thet joinj in conditioniti inin thet WHERE clausel ..

•• Prefixr fix thet columnl name withith thet tablet le name when thet same columnl name appearsrs inin morere thant one tablet le..

4-4

Cartesiani Product

•• A Cartesianrt i productr t isis formedf r when::

–A joinj in conditioniti isis omitteditt

–A joinj in conditioniti isis invalidi lid

–Allll rowsr inin thet firstfir t tablet le arere joinedj i toto allll rowsr inin thet second tablet le

•• To avoidid a Cartesianrt i product,r t, alwaysl includei l a validlid joinj in conditioniti inin a WHERE clausel ..

4-5

Generatingi a Cartesiani Product

4-6

Types of Joinsi

Equijoinij in Non--equijoinij in Outert r joinj in Selflf joinj in

4-7

What IsIs an Equijoin?ij i

4-8

Retrievingi i Records

withi Equijoinsij i

14 rows selected.

4-9

Qualifyingli i Ambiguousi Columnl Names

•• Use tablet le prefixesr fi toto qualifylify columnl names thatt t arere inin multiplelti le tablest l ..

•• ImproveI r performancerf r by usingi tablet le prefixesr fi ..

•• Distinguishi ti i columnsl thatt t have identicali ti l names butt resider i inin differentiff r t tablest l by usingi columnl aliasesli ..

4-10

Additionali i l Search Conditionsi i

Usingi the AND Operator

4-11