CROSS JOIN

一，背景：网上找关于查看SQL执行计划的SQL时，发现这样一条SQL:

SELECT cp.usecounts as '使用次数'

            ,objtype as '类型'

            ,st.text            

FROM sys.dm_exec_cached_plans cp

CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS st

WHERE st.text not like '%sys%'

     这句SQL（’ CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS st'）神奇了，竟然能够连接带参数的函数（参数不是固定的值，是另一张表的某个字段）。而正常我们如果像这样’ CROSS JOIN sys.dm_exec_sql_text(plan_handle) AS st’连接时，会得到下面的错误’ The multi-part identifier " plan_handle " could not be bound.’.。

二，OSS JOIN详解

Apply操作符的功能:
    可以让select查询语法与表值函数(TVF)进一步结合,在数据表含有XML字段时,结合XML的内容转换为新的数据表字段颇为方便.
                                                                                                                                        
Apply 工作原理:
   Apply操作符让符合查询的每一条记录都调用一次TVF函数,并将结果与原数据表的记录内容一起展开.
   Apply操作符定义在From子句内,使用方式与Join操作符类似. 其格式有两种类型:
   CROSS APPLY
   OUTER APPLY

       两者的差异只在当外部的数据表逐条将记录带入到TVF之后,若TVF并无符合的记录返回时, CROSS APPLY运算将不会显示该条记录,而OUTER APPLY依然会显示外部数据表对应的记录,但TVF相关的字段则以NULL显示.

三，示例

     初始化数据（下面仅仅是展示下CROSS APPLY的应用，例子举的差）：

View Code

IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[Dept]'))

DROP TABLE [dbo].[Dept]

GO

CREATE TABLE dbo.Dept(

    id INT,

    name VARCHAR(50)

)

GO



 

INSERT INTO dbo.Dept

SELECT 1,'软件工程学院'

UNION ALL

SELECT 2,'美术学院'

UNION ALL

SELECT 3,'外语学院'



SELECT * FROM dbo.Dept



IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[Employee]'))

DROP TABLE [dbo].[Employee]

GO

CREATE TABLE dbo.Employee(

    id INT,

    name VARCHAR(50),

    dept_id INT

)

GO

INSERT INTO dbo.Employee

SELECT 1,'Mike',1

UNION ALL

SELECT 2,'Ross',1

UNION ALL

SELECT 3,'Jion',2



SELECT * FROM dbo.Employee

    

    创建多函数，并使用CROSS JOIN 与OUTER JOIN:

View Code

IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[GetEployeeCountByDeptID]'))

DROP FUNCTION [dbo].[GetEployeeCountByDeptID]

GO



CREATE FUNCTION dbo.GetEployeeCountByDeptID

(

   @dept_id INT

)

RETURNS TABLE

AS

RETURN

(

   SELECT E.dept_id, COUNT(*) AS employee_num FROM Employee AS E 

   WHERE E.dept_id=@dept_id

   GROUP BY E.dept_id

)



GO



 

SELECT *

FROM dbo.Dept AS D

 CROSS APPLY dbo.GetEployeeCountByDeptID(D.id)



SELECT *

FROM dbo.Dept AS D

 OUTER APPLY dbo.GetEployeeCountByDeptID(D.id)