SQL 按照两个字段去除列重复的数据，保留一行。

首先创建一个例子要用的表

  CREATE TABLE [dbo].[as1]
(
[id] [nchar] (10) COLLATE Chinese_PRC_CI_AS NULL,
[a] [nchar] (10) COLLATE Chinese_PRC_CI_AS NULL,
[b] [nchar] (10) COLLATE Chinese_PRC_CI_AS NULL
) ON [PRIMARY]

GO

插入例子要用的数据

INSERT INTO dbo.as1
        ( id, a, b )
VALUES  ( N'1', -- id - nchar(10)
          N'2', -- a - nchar(10)
          N'3'  -- b - nchar(10)
          ),
   ( N'2', -- id - nchar(10)
          N'2', -- a - nchar(10)
          N'2'  -- b - nchar(10)
          ),
  ( N'3', -- id - nchar(10)
          N'2', -- a - nchar(10)
          N'1'  -- b - nchar(10)
          ),
  ( N'4', -- id - nchar(10)
          N'2', -- a - nchar(10)
          N'2'  -- b - nchar(10)
          ),
  ( N'5', -- id - nchar(10)
          N'1', -- a - nchar(10)
          N'2'  -- b - nchar(10)
          )

-----------分割线---------

 SELECT *  FROM dbo.as1

--简单查看全部数据

   SELECT a.a,a.b FROM dbo.as1 AS a  JOIN dbo.as1 AS T ON T.id = a.id WHERE a.a= T.a AND a.b= T.b 

 GROUP BY a.a, a.b  HAVING COUNT(*) > 1

 --查询出重复的数据

 SELECT id FROM dbo.as1 T
WHERE EXISTS 
(
SELECT 1 FROM dbo.as1 WHERE a= T.a AND b= T.b 
 GROUP BY a, b  HAVING COUNT(*) > 1

 )

 --查询出重复数据对应的ID号

 SELECT min(id)AS 最小的ID FROM dbo.as1 GROUP BY a, b HAVING COUNT(*) > 1

 --查询出数据里重复ID的最小值(准备保留大的ID)

--下面是要删除的语句的查询

SELECT *  FROM dbo.as1
WHERE id IN


(
SELECT id FROM dbo.as1 T


WHERE EXISTS 
(
SELECT 1 FROM dbo.as1 WHERE a= T.a AND b= T.b 
 GROUP BY a, b  HAVING COUNT(*) > 1
 )
 
 
AND
 T.id NOT IN
 (
 SELECT min(id) FROM dbo.as1 GROUP BY a, b HAVING COUNT(*) > 1
 )

 )

--删除语句

DELETE FROM   dbo.as1
WHERE id IN


(
SELECT id FROM dbo.as1 T


WHERE EXISTS 
(
SELECT 1 FROM dbo.as1 WHERE a= T.a AND b= T.b 
 GROUP BY a, b  HAVING COUNT(*) > 1
 )
 
 
AND
 T.id NOT IN
 (
 SELECT min(id) FROM dbo.as1 GROUP BY a, b HAVING COUNT(*) > 1
 )

 )

--------------

DELETE FROM  表名
WHERE ID号 IN


(
SELECT  ID号 FROM 表名 T


WHERE EXISTS 
(
SELECT 1 FROM 表名 WHERE 字段1= T.字段1 AND 字段2= T.字段2 
 GROUP BY 字段1, 字段2  HAVING COUNT(*) > 1
 )
 
 
AND
 T. ID号 NOT IN
 (
 SELECT min( ID号) FROM 表名GROUP BY 字段1, 字段2 HAVING COUNT(*) > 1
 )
 )