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
结果11：
a_id a_value b_id b_value c_id c_value
-----------------------------------------------------
1 1 1 1 1 1
2 1 2 3 2 2
3 2 NULL NULL NULL NULL
结果12：
a_id a_value b_id b_value c_id c_value
-----------------------------------------------------
1 1 1 1 1 1
2 1 NULL NULL NULL NULL
3 2 NULL NULL NULL NULL
结果13：
a_id a_value b_id b_value c_id c_value
-----------------------------------------------------
1 1 1 1 1 1
2 1 2 3 NULL NULL
3 2 NULL NULL NULL NULL
结果14：
a_id a_value b_id b_value c_id c_value
-----------------------------------------------------
1 1 1 1 NULL NULL
2 1 2 3 2 2
3 2 NULL NULL NULL NULL
通过以上三表数据查询结果，可以看出，LEFT JOIN 查询，对于ON的单独表条件始终只会影响条件表的右表（如，a.value=1会影响b表关联的a表value字段值为1的行，并不会限制a表的数据只显示value=1的行），RIGHT JOIN 影响效果恰恰相反
在使用ON条件时LEFT JOIN影响的是右侧的第二张第三张表，并不会对最左侧的表影响，所以对于a,b,c,三张表，a表数据是不受ON条件影响的，只会影响联接查询后的b或c数据
而WHERE就相当于在WHERE条件之前查询的数据当着一个表，然后通过WHERE条件进行筛选数据，所以影响的是整体。
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