最近在講到T-SQL查詢的Join部分時，一下子沒有想起來CROSS JOIN的用法，因為其實平常也確實基本不用到。特意找了一個例子，以供參考

CROSS JOIN又稱為笛卡爾乘積，實際上是把兩個表乘起來。以下資料摘自：http://www.sqlguides.com/sql_cross_join.php

SQL CROSS JOIN will return all records where each row from the first table is combined with each row from the second table. Which also mean CROSS JOIN returns the Cartesian product of the sets of rows from the joined tables.

A CROSS JOIN can be specified in two ways: using the JOIN syntax or by listing the tables in the FROM clause separated by commas without using a WHERE clause to supply join criteria.

SQL CROSS JOIN syntax:

SELECT * FROM [TABLE 1] CROSS JOIN [TABLE 2]

OR

SELECT * FROM [TABLE 1], [TABLE 2]

EXAMPLE :

Let's try with 2 tables below:

Table 1: GameScores

PlayerName	DepartmentId	Scores
Jason	1	3000
Irene	1	1500
Jane	2	1000
David	2	2500
Paul	3	2000
James	3	2000

Table 2: Departments

DepartmentId	DepartmentName
1	IT
2	Marketing
3	HR

SQL statement :

SELECT* FROM GameScores CROSS JOIN Departments

Result:

?

PlayerName	DepartmentId	Scores	DepartmentId	DepartmentName
Jason	1	3000	1	IT
Irene	1	1500	1	IT
Jane	2	1000	1	IT
David	2	2500	1	IT
Paul	3	2000	1	IT
James	3	2000	1	IT
Jason	1	3000	2	Marketing
Irene	1	1500	2	Marketing
Jane	2	1000	2	Marketing
David	2	2500	2	Marketing
Paul	3	2000	2	Marketing
James	3	3000	2	Marketing
Jason	1	3000	3	HR
Irene	1	1500	3	HR
Jane	2	1000	3	HR
David	2	2500	3	HR
Paul	3	2000	3	HR
James	3	3000	3	HR

總結

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