1. Chapter 4 Joining Multiple Tables

2. Chapter Objectives Create a Cartesian join
Create an equality join using the WHERE clause
Create an equality join using the JOIN keyword
Create a non-equality join using the WHERE clause
Create a non-equality join using the JOIN…ON approach

3. Chapter Objectives Create a self-join
Distinguish an inner join from an outer join
Create an outer join using the WHERE clause
Create an outer join using the OUTER keyword
Use set operators to combine the results of multiple queries
Join three or more tables

4. Purpose of Joins
Joins are used to link tables and reconstruct data in a relational database
Joins can be created through:
Conditions in a WHERE clause
Use of JOIN keywords in FROM clause

5. Cartesian Join
Created by omitting joining condition in the WHERE clause or through CROSS JOIN keywords in the FROM clause
Results in every possible row combination (m * n)

6. Cartesian Join Example: Omitted Condition

7. Cartesian Join Example: CROSS JOIN Keywords

8. Equality Join
Links rows through equivalent data that exists in both tables
Created by:
Creating equivalency condition in the WHERE clause
Using NATURAL JOIN, JOIN…USING, or JOIN…ON keywords in the FROM clause

9. JOIN Keyword Overview
Use NATURAL JOIN when tables have one column in common
Use JOIN…USING when tables have more than one column in common
Use JOIN…ON when a condition is needed to specify a relationship other than equivalency
Using JOIN keyword frees the WHERE clause for exclusive use in restricting rows

10. Equality Join: WHERE Clause Example

11. Equality Join: NATURAL JOIN
Syntax: tablename NATURAL JOIN tablename

12. Equality Join: JOIN…USING
Syntax: tablename JOIN tablename USING (columnname)

13. Equality Join: JOIN…ON
Syntax: tablename JOIN tablename ON condition

14. Non-Equality Joins
In WHERE clause, use any comparison operator other than equal sign
In FROM clause, use JOIN…ON keywords with non-equivalent condition

15. Non-Equality Join: WHERE Clause Example

16. Non-Equality Join: JOIN…ON Example

17. Self-Joins Used to link a table to itself
Requires use of column qualifier

18. Self-Join: WHERE Clause Example

19. Self-Join: JOIN…ON Example

20. Outer Joins
Use to include rows that do not have a match in the other table
In WHERE clause, include outer join operator (+) next to table with missing rows to add NULL rows
In FROM clause, use FULL, LEFT, or RIGHT with OUTER JOIN keywords

21. Outer Join: WHERE Clause Example

22. Outer Join: OUTER JOIN Keyword Example

23. Set Operators
Used to combine the results of two or more SELECT statements

24. Set Operator Example

25. Joining Three or More Tables
Same procedure as joining two tables
Will always results in one less join than the number of tables being joined

26. Joining Three or More Tables: Example