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關于SQL SERVER的表聯接查詢INNER JOIN 、LEFT JOIN和RIGHT JOIN，經常會用到ON和WHERE的條件查詢，以前用的時候有時是憑感覺的，總是沒有搞清楚，今日親自測試了下，理解到了一些內容，在此分享。
要測試，首先我們來創建三張表，數據庫就根據自己的情況而定
創建表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 ONGOSET QUOTED_IDENTIFIER ONGOCREATE 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 ONGOSET QUOTED_IDENTIFIER ONGOCREATE TABLE [dbo].[TestJoinOnOrWhere_C]( [id] [int] NULL, [value] [int] NULL) ON [PRIMARY]
表創建好了然后我們添加幾條數據
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)
現在我們開始測試
語句1：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND a.value = 1語句2：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id
結果1：
id value id value
-------------------------------
1 1 1 1
2 1 2 3
3 2 NULL NULL
結果2
id value id value
-------------------------------
1 1 1 1
2 1 2 3
3 2 3 4
在網上查詢到，有的人說a.value = 1沒有生效，其實不然，它已經生效，只是在左聯接查詢時，左表的數據是不會受影響，只有右表的數據會根據a.value = 1條件取出左表(a表)Value為1的行，通過上面兩個語句的結果就可以看出，那么我們用右表篩選條件會出現什么呢？看看下面語句
語句3：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND b.value = 1
結果3：
id value id value
-------------------------------
1 1 1 1
2 1 NULL NULL
3 2 NULL NULL
以上結果看出，也只是影響了右表的數據
語句4：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.value = 1
結果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
從上面語句結果看出，也只影響了右表的數據（取出所有a表value對應為1的b表數據）
所以在左聯接查詢時ON后面的條件只會影響右表，相反右聯接查詢影響的就是左邊的表數據
如果用WHERE呢？我們看下下面的語句
語句5：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id where a.value = 1語句6：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a LEFT JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id where b.value = 1
結果5:
id value id value
-------------------------------
1 1 1 1
2 1 2 3
結果6:
id value id value
-------------------------------
1 1 1 1
可以從結果看出，這個影響的結果就是全部的表，就相當于通過ON條件聯接查詢查詢的結果，然后通過WHERE后面的條件取總體篩選
對于INNER JOIN 的ON條件會怎樣影響呢？先看下面語句執行結果
語句7： SELECT * FROM dbo.TestJoinOnOrWhere_A AS a INNER JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND a.value = 1語句8： SELECT * FROM dbo.TestJoinOnOrWhere_A AS a INNER JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id AND b.value = 1語句9： SELECT * FROM dbo.TestJoinOnOrWhere_A AS a INNER JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id WHERE a.value = 1語句10：SELECT * FROM dbo.TestJoinOnOrWhere_A AS a INNER JOIN dbo.TestJoinOnOrWhere_B AS b ON a.id = b.id WHERE b.value = 1
結果7/9:
id value id value
-------------------------------
1 1 1 1
2 1 2 3
結果8/10:
id value id value
-------------------------------
1 1 1 1
上面通過WHERE和ON查詢出來的結果是一樣的，由此可看出，INNER JOIN 的ON條件和WHERE條件影響的都是一個效果，影響整體的查詢結果。
下面我們再來看下對于LEFT JOIN的三表查詢對于WHERE和ON影響的結果
語句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
語句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
語句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
語句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
結果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
結果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
結果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
結果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
通過以上三表數據查詢結果，可以看出，LEFT JOIN 查詢，對于ON的單獨表條件始終只會影響條件表的右表（如，a.value=1會影響b表關聯的a表value字段值為1的行，并不會限制a表的數據只顯示value=1的行），RIGHT JOIN 影響效果恰恰相反
在使用ON條件時LEFT JOIN影響的是右側的第二張第三張表，并不會對最左側的表影響，所以對于a,b,c,三張表，a表數據是不受ON條件影響的，只會影響聯接查詢后的b或c數據
而WHERE就相當于在WHERE條件之前查詢的數據當著一個表，然后通過WHERE條件進行篩選數據，所以影響的是整體。
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