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  • CROSS JOIN

    最近在讲到T-SQL查询的Join部分时,一下子没有想起来CROSS JOIN的用法,因为其实平常也确实基本不用到。特意找了一个例子,以供参考

    CROSS JOIN又称为笛卡尔乘积,实际上是把两个表乘起来。以下资料摘自: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
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  • 原文地址:https://www.cnblogs.com/chenxizhang/p/1330325.html
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