delete from 表
where id not in(select min(id) from 表 group by name ) //删除重复名字的记录

删除之前请用语句
select *
from 表
where id in(select min(id) from 表 group by name )
查看能保留下来的数据。

eg.delete from T_bbs_subject
where subjectId not in(select min(subjectId) from T_bbs_subject group by clsid )

几个删除重复记录的SQL语句

(1)用group by方法

查数据:
  select count(num), max(name) from student --列出重复的记录数,并列出他的name属性 
  group by num 
  having count(num) >1 --按num分组后找出表中num列重复,即出现次数大于一次 
删数据:
  delete from student 
  group by num 
  having count(num) >1
  这样的话就把所有重复的都删除了。

查询重复

select * from tablename where id in (select id from tablename group by id having count(id) > 1)

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)
 
比方说
在A表中存在一个字段“name”,
而且不同记录之间的“name”值有可能会相同,
现在就是需要查询出在该表中的各记录之间,“name”值存在重复的项;
Select Name,Count(*) From A Group By Name Having Count(*) > 1

如果还查性别也相同大则如下:
Select Name,sex,Count(*) From A Group By Name,sex Having Count(*) > 1
 
"重复记录"有两个意义上的重复记录,一是完全重复的记录,也即所有字段均重复的记录,二是部分关键字段重复的记录,比如Name字段重复,而其他字段不一定重复或都重复可以忽略。
 
1、对于第一种重复,比较容易解决,使用

select distinct * from tableName
  就可以得到无重复记录的结果集。
  如果该表需要删除重复的记录(重复记录保留1条),可以按以下方法删除
select distinct * into #Tmp from tableName
drop table tableName
select * into tableName from #Tmp
drop table #Tmp
  发生这种重复的原因是表设计不周产生的,增加唯一索引列即可解决。
 
2、这类重复问题通常要求保留重复记录中的第一条记录,操作方法如下

  假设有重复的字段为Name,Address,要求得到这两个字段唯一的结果集
select identity(int,1,1) as autoID, * into #Tmp from tableName
select min(autoID) as autoID into #Tmp2 from #Tmp group by Name,Address,autoID
select * from #Tmp where autoID in(select autoID from #tmp2)
  最后一个select即得到了Name,Address不重复的结果集(但多了一个autoID字段,实际写时可以写在select子句中省去此列), 用select identity(int,1,1) as id,* into #temp保存到一张临时表中,然后就可以对临时表#temp进行操作了
 
 
--I、Name相同ID最小的记录,方法在SQl05时,效率高于、
方法:
Select * from #T a where not exists(select from #T where Name=a.Name and ID<a.ID) 方法:
select a.* from #T a join (select min(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID 方法:
select * from #T a where ID=(select min(ID) from #T where Name=a.Name) 方法:
select a.* from #T a join #T b on a.Name=b.Name and a.ID>=b.ID group by a.ID,a.Name,a.Memo having count()= 方法:
select * from #T a group by ID,Name,Memo having ID=(select min(ID)from #T where Name=a.Name) 方法:
select * from #T a where (select count() from #T where Name=a.Name and ID<a.ID)= 方法:
select * from #T a where ID=(select top ID from #T where Name=a.name order by ID) 方法:
select * from #T a where ID!>all(select ID from #T where Name=a.Name) 方法(注:ID为唯一时可用):
select * from #T a where ID in(select min(ID) from #T group by Name) --SQL2005: 方法:
select ID,Name,Memo from (select *,min(ID)over(partition by Name) as MinID from #T a)T where ID=MinID 方法: select ID,Name,Memo from (select *,row_number()over(partition by Name order by ID) as MinID from #T a)T where MinID=
--II、Name相同ID最大的记录,与min相反:
方法:
Select * from #T a where not exists(select from #T where Name=a.Name and ID>a.ID) 方法:
select a.* from #T a join (select max(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID order by ID 方法:
select * from #T a where ID=(select max(ID) from #T where Name=a.Name) order by ID 方法:
select a.* from #T a join #T b on a.Name=b.Name and a.ID<=b.ID group by a.ID,a.Name,a.Memo having count()= 方法:
select * from #T a group by ID,Name,Memo having ID=(select max(ID)from #T where Name=a.Name) 方法:
select * from #T a where (select count() from #T where Name=a.Name and ID>a.ID)= 方法:
select * from #T a where ID=(select top ID from #T where Name=a.name order by ID desc) 方法:
select * from #T a where ID!<all(select ID from #T where Name=a.Name) 方法(注:ID为唯一时可用):
select * from #T a where ID in(select max(ID) from #T group by Name) --SQL2005: 方法:
select ID,Name,Memo from (select *,max(ID)over(partition by Name) as MinID from #T a)T where ID=MinID 方法:
select ID,Name,Memo from (select *,row_number()over(partition by Name order by ID desc) as MinID from #T a)T where MinID=

删除重复记录有大小关系时,保留大或小其中一个记录

--I、Name相同ID最小的记录,保留最小一条
方法:
delete a from #T a where exists(select from #T where Name=a.Name and ID<a.ID) 方法:
delete a from #T a left join (select min(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID where b.Id is null 方法:
delete a from #T a where ID not in (select min(ID) from #T where Name=a.Name) 方法(注:ID为唯一时可用):
delete a from #T a where ID not in(select min(ID)from #T group by Name) 方法:
delete a from #T a where (select count() from #T where Name=a.Name and ID<a.ID)> 方法:
delete a from #T a where ID<>(select top ID from #T where Name=a.name order by ID) 方法:
delete a from #T a where ID>any(select ID from #T where Name=a.Name)
--II、Name相同ID保留最大的一条记录:

方法:
delete a from #T a where exists(select from #T where Name=a.Name and ID>a.ID) 方法:
delete a from #T a left join (select max(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID where b.Id is null 方法:
delete a from #T a where ID not in (select max(ID) from #T where Name=a.Name) 方法(注:ID为唯一时可用):
delete a from #T a where ID not in(select max(ID)from #T group by Name) 方法:
delete a from #T a where (select count() from #T where Name=a.Name and ID>a.ID)> 方法:
delete a from #T a where ID<>(select top ID from #T where Name=a.name order by ID desc) 方法:
delete a from #T a where ID<any(select ID from #T where Name=a.Name)
--、删除重复记录没有大小关系时,处理重复值
方法:
if object_id('Tempdb..#') is not null
drop table #
Select distinct * into # from #T--排除重复记录结果集生成临时表# truncate table #T--清空表 insert #T select * from # --把临时表#插入到表#T中 --查看结果
select * from #T --重新执行测试数据后用方法
方法: alter table #T add ID int identity--新增标识列
go
delete a from #T a where exists(select from #T where Num=a.Num and Name=a.Name and ID>a.ID)--只保留一条记录
go
alter table #T drop column ID--删除标识列 --查看结果
select * from #T
 

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