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删除Table表中的重复行的方法

2019-01-04 MsSQL 前端之家

前端之家收集整理的这篇文章主要介绍了删除Table表中的重复行的方法，前端之家小编觉得挺不错的，现在分享给大家，也给大家做个参考。

利用sql Server 2005的新功能NOWNUMBER和CTE可以很好的实现.
举例说明如下:
建立测试数据:
<div class="codetitle">@L502_0@ 代码如下:

<div class="codebody" id="code42033">
create table Dup1
(
Col1 int null,
Col2 varchar(20) null
)
insert into Dup1 values
(1,'aaa'),
(2,
(3,'bbb'),
(4,'ccc'),'ddd'),
(5,'eee')
select from Dup1

可以查看到重复的数据有:
<div class="codetitle"><a style="CURSOR: pointer" data="33747" class="copybut" id="copybut33747" onclick="doCopy('code33747')"> 代码如下:

<div class="codebody" id="code33747">
SELECT Col1,Col2,COUNT() AS DupCountFROM Dup1GROUP BY Col1,Col2HAVING COUNT() > 1

<IMG src="http:https://files.jb51.cc/upload/201006/20100608002211701.jpg">
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.
<div class="codetitle"><a style="CURSOR: pointer" data="11720" class="copybut" id="copybut11720" onclick="doCopy('code11720')"> 代码如下:

<div class="codebody" id="code11720">
SELECT Col1,ROW_NUMBER() OVER (PARTITION BY Col1,Col2 ORDER BY Col1) AS rnFROM Dup1

得到的序号如下:
<IMG src="http:https://files.jb51.cc/upload/201006/20100608002211200.jpg">
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.
<div class="codetitle"><a style="CURSOR: pointer" data="51241" class="copybut" id="copybut51241" onclick="doCopy('code51241')"> 代码如下:

<div class="codebody" id="code51241">
--用到CTE
WITH DupsD
AS (
SELECT Col1,
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 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.
<div class="codetitle"><a style="CURSOR: pointer" data="32678" class="copybut" id="copybut32678" onclick="doCopy('code32678')"> 代码如下:<div class="codebody" id="code32678">
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.
<div class="codetitle"><a style="CURSOR: pointer" data="14925" class="copybut" id="copybut14925" onclick="doCopy('code14925')"> 代码如下:<div class="codebody" id="code14925">
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.
<div class="codetitle"><a style="CURSOR: pointer" data="41628" class="copybut" id="copybut41628" onclick="doCopy('code41628')"> 代码如下:<div class="codebody" id="code41628">
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.

重复行