利用SQL Server 2005的新功能NOW_NUMBER和CTE可以很好的实现.

举例说明如下:

建立测试数据:

复制代码 代码如下:

create table Dup1

(

Col1 int null,

Col2 varchar(20) null

)

insert into Dup1 values

(1, 'aaa'),

(2, 'aaa'),

(2, 'aaa'),

(2, 'aaa'),

(3, 'bbb'),

(3, 'bbb'),

(4, 'ccc'),

(4, 'ddd'),

(5, 'eee')

select * from Dup1

可以查看到重复的数据有:

复制代码 代码如下:

SELECT Col1, Col2, COUNT(*) AS DupCountFROM Dup1GROUP BY Col1, Col2HAVING COUNT(*) > 1

接下来介绍如何delete掉重复的数据:

1.NOW_NUMBER:SQL Server 2005添加了很好用的RANKING函数(NOW_NUMBER,RANK,DENSE_RANK,NTILE),利用NOW_NUMBER()OVER(PARTITION GY)最为直接,也最为方便,不能修改表或者产生多余的列.

首先会分配一个列号码,以Col1,Col2组合来分区排序.

复制代码 代码如下:

SELECT Col1, Col2,ROW_NUMBER() OVER (PARTITION BY Col1, Col2 ORDER BY Col1) AS rnFROM Dup1

得到的序号如下:

很明显的是重复列都分组分割排序,只需要delete掉排序序号>1的即可.

复制代码 代码如下:

--用到CTE

WITH DupsD

AS (

SELECT Col1, Col2,

ROW_NUMBER() OVER (PARTITION BY Col1, Col2 ORDER BY Col1) AS rn

FROM Dup1

)

DELETE DupsD

WHERE rn > 1;

--或者

DELETE A FROM (

SELECT Col1, Col2,

ROW_NUMBER() OVER (PARTITION BY Col1, Col2 ORDER BY Col1) AS rn

FROM Dup1) A WHERE A.rn>1

2.创建一个标识键唯一的表记一列.

复制代码 代码如下:

ALTER TABLE dbo.Dup1

ADD

PK INT IDENTITY

NOT NULL

CONSTRAINT PK_Dup1 PRIMARY KEY;

SELECT *

FROM Dup1;

删除找出与Col1,Col2相同并且比Dup1.PK大的记录,也就是保留重复值中PK最小的记录.

复制代码 代码如下:

DELETE Dup1

WHERE EXISTS ( SELECT *

FROM Dup1 AS D1

WHERE D1.Col1 = Dup1.Col1

AND D1.Col2 = Dup1.Col2

AND D1.PK > Dup1.PK );

3.select distant into,这种方法借助一个新的table,把不重复的结果集转移到新table中.

复制代码 代码如下:

SELECT distinct Col1, Col2 INTO NoDupsFROM Dup1;select * from NoDups

建议采用第一种和第三种方法,第一种多见于T-SQL的编程中,第三种在ETL中常常使用.