关于SQL删除重复数据只保留一条

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本文详细介绍了使用SQL语句删除数据库中重复记录的方法，包括单个字段和多个字段的情况，提供了多种实现方式及其优缺点分析，旨在帮助数据库管理员和开发者高效地解决数据去重问题。

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用SQL语句,删除掉重复项只保留一条

在几千条记录里,存在着些相同的记录,如何能用SQL语句,删除掉重复的呢
1、查找表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断
select * from people
where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)

2、删除表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断，只留有rowid最小的记录
delete from people
where peopleName in (select peopleName from people group by peopleName having count(peopleName) > 1)
and peopleId not in (select min(peopleId) from people group by peopleName having count(peopleName)>1)

3、删除表中多余的重复记录，重复记录是根据对个字段来判断的。并且重复条数多余2条。

while 1=1
begin
delete top (1) from CNLaw_Info
from CNLaw_Info p1 join ( select app_no,noticeDate,law_state,law_info from CNLaw_Info group by app_no,noticeDate,law_state
,law_info having COUNT(*)>1 )
p2 on p1.app_no=p2.app_no and p1.noticeDate=p2.noticeDate and p1.law_state=p2.law_state and p1.law_info=p2.law_info
if @@ROWCOUNT=0 break;
end;

上面这种方法效率非常的低，后来通过改进，例子如下：

select * from Patent.dbo.patent_US_Grant where ID in(select max(ID)
from Patent.dbo.patent_US_Grant where NoticeDate='20121023' group by ApplicationNumber having COUNT(ApplicationNumber)>1)

如果要删除的话把 “select *” 改为delete

解析上面语句：其实在sql server 最好用上 exists关键字，在这里的后面的select max(ID)。。。，查询出id最大的记录。

如果我们在查询时候，不用到having count 那么多个字段相同，找出重复数据的sql：

比如现在有一人员表（表名：peosons）
若想将姓名、身份证号、住址这三个字段完全相同的记录查询出来：select p1.* from persons p1,persons p2 where p1.id<>p2.id and p1.cardid = p2.cardid and p1.pname = p2.pname and p1.address = p2.address

3、查找表中多余的重复记录（多个字段）
select * from vitae a
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)

4、删除表中多余的重复记录（多个字段），只留有rowid最小的记录
delete from vitae a
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)
and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1)

5、查找表中多余的重复记录（多个字段），不包含rowid最小的记录
select * from vitae a
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)
and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1)

6.消除一个字段的左边的第一位：

update tableName set [Title]=Right([Title],(len([Title])-1)) where Title like '村%'

7.消除一个字段的右边的第一位：

update tableName set [Title]=left([Title],(len([Title])-1)) where Title like '%村'

8.假删除表中多余的重复记录（多个字段），不包含rowid最小的记录
update vitae set ispass=-1
where peopleId in (select peopleId from vitae group by peopleId