sql语句中left join和inner join中的on与where的区别分析

原文:sql语句中left join和inner join中的on与where的区别分析

关于SQL SERVER的表联接查询INNER JOIN 、LEFT JOIN和RIGHT JOIN，经常会用到ON和WHERE的条件查询，以前用的时候有时是凭感觉的，总是没有搞清楚，今日亲自测试了下，理解到了一些内容，在此分享。

要测试，首先我们来创建三张表，数据库就根据自己的情况而定

创建表TestJoinOnOrWhere_A、TestJoinOnOrWhere_B、TestJoinOnOrWhere_C

/****** Object:  Table [dbo].[TestJoinOnOrWhere_A]    Script Date: 2015/4/3 14:34:41 ******/
CREATE TABLE [dbo].[TestJoinOnOrWhere_A]([id] [int] NULL,[value] [int] NULL
) ON [PRIMARY]GO
/****** Object:  Table [dbo].[TestJoinOnOrWhere_B]    Script Date: 2015/4/3 14:34:41 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[TestJoinOnOrWhere_B]([id] [int] NULL,[value] [int] NULL
) ON [PRIMARY]GO
/****** Object:  Table [dbo].[TestJoinOnOrWhere_C]    Script Date: 2015/4/3 14:34:41 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[TestJoinOnOrWhere_C]([id] [int] NULL,[value] [int] NULL
) ON [PRIMARY]

表创建好了然后我们添加几条数据

INSERT [dbo].[TestJoinOnOrWhere_A] ([id], [value]) VALUES (1, 1)
INSERT [dbo].[TestJoinOnOrWhere_A] ([id], [value]) VALUES (2, 1)
INSERT [dbo].[TestJoinOnOrWhere_A] ([id], [value]) VALUES (3, 2)
INSERT [dbo].[TestJoinOnOrWhere_B] ([id], [value]) VALUES (1, 1)
INSERT [dbo].[TestJoinOnOrWhere_B] ([id], [value]) VALUES (2, 3)
INSERT [dbo].[TestJoinOnOrWhere_B] ([id], [value]) VALUES (3, 4)
INSERT [dbo].[TestJoinOnOrWhere_C] ([id], [value]) VALUES (1, 1)
INSERT [dbo].[TestJoinOnOrWhere_C] ([id], [value]) VALUES (2, 2)
INSERT [dbo].[TestJoinOnOrWhere_C] ([id], [value]) VALUES (3, 3)

现在我们开始测试

语句1：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND a.value = 1
语句2：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id

结果1：

id value id value

-------------------------------
1 1 1 1
2 1 2 3
3 2 NULL NULL

结果2

id value id value

-------------------------------
1 1 1 1
2 1 2 3
3 2 3 4

在网上查询到，有的人说a.value = 1没有生效，其实不然，它已经生效，只是在左联接查询时，左表的数据是不会受影响，只有右表的数据会根据a.value = 1条件取出左表(a表)Value为1的行，通过上面两个语句的结果就可以看出，那么我们用右表筛选条件会出现什么呢？看看下面语句

语句3：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND b.value = 1

结果3：

id value id value

-------------------------------
1 1 1 1
2 1 NULL NULL
3 2 NULL NULL

以上结果看出，也只是影响了右表的数据

语句4：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON  a.value = 1

结果4：

id value id value

-------------------------------
1 1 1 1
1 1 2 3
1 1 3 4
2 1 1 1
2 1 2 3
2 1 3 4
3 2 NULL NULL

从上面语句结果看出，也只影响了右表的数据（取出所有a表value对应为1的b表数据）

所以在左联接查询时ON后面的条件只会影响右表，相反右联接查询影响的就是左边的表数据

如果用WHERE呢？我们看下下面的语句

语句5：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id where a.value = 1语句6：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id where b.value = 1

结果5:

id value id value

-------------------------------
1 1 1 1
2 1 2 3

结果6:

id value id value

-------------------------------
1 1 1 1

可以从结果看出，这个影响的结果就是全部的表，就相当于通过ON条件联接查询查询的结果，然后通过WHERE后面的条件取总体筛选

对于INNER JOIN 的ON条件会怎样影响呢？先看下面语句执行结果

语句7：  SELECT * FROM dbo.TestJoinOnOrWhere_A AS a INNER JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND a.value = 1
语句8：  SELECT * FROM dbo.TestJoinOnOrWhere_A AS a INNER JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND b.value = 1
语句9：  SELECT * FROM dbo.TestJoinOnOrWhere_A AS a INNER JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id WHERE a.value = 1
语句10：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a INNER JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id WHERE b.value = 1

结果7/9:

id value id value

-------------------------------
1 1 1 1
2 1 2 3

结果8/10:

id value id value

-------------------------------
1 1 1 1

上面通过WHERE和ON查询出来的结果是一样的，由此可看出，INNER JOIN 的ON条件和WHERE条件影响的都是一个效果，影响整体的查询结果。

下面我们再来看下对于LEFT JOIN的三表查询对于WHERE和ON影响的结果

语句11：SELECT a.id AS a_id,a.value AS a_value,b.id AS b_id,b.value AS b_value,c.id AS c_id,c.value AS c_value FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND a.value = 1 LEFT JOIN dbo.TestJoinOnOrWhere_C AS c ON b.id = c.id
语句12：SELECT a.id AS a_id,a.value AS a_value,b.id AS b_id,b.value AS b_value,c.id AS c_id,c.value AS c_value FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND b.value = 1 LEFT JOIN dbo.TestJoinOnOrWhere_C AS c ON b.id = c.id
语句13：SELECT a.id AS a_id,a.value AS a_value,b.id AS b_id,b.value AS b_value,c.id AS c_id,c.value AS c_value FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND a.value = 1 LEFT JOIN dbo.TestJoinOnOrWhere_C AS c ON b.id = c.id AND b.value = 1
语句14：SELECT a.id AS a_id,a.value AS a_value,b.id AS b_id,b.value AS b_value,c.id AS c_id,c.value AS c_value FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND a.value = 1 LEFT JOIN dbo.TestJoinOnOrWhere_C AS c ON b.id = c.id AND c.value = 2