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 tablenamegroup by idhaving count(id) > 1)
查找表中多余的重复记录（多个字段）
select * from vitae awhere (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selectidentity(int,1,1)asid,*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 1 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(1)=1 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(1) from #T where Name=a.Name and ID<a.ID)=0方法:select * from #T a where ID=(select top 1 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)--SQL: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=1
--II、Name相同ID最大的记录,与min相反:方法:Select * from #T a where not exists(select 1 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(1)=1 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(1) from #T where Name=a.Name and ID>a.ID)=0方法:select * from #T a where ID=(select top 1 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)--SQL: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=1
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 1 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(1) from #T where Name=a.Name and ID<a.ID)>0方法:delete a from #T a where ID<>(select top 1 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 1 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(1) from #T where Name=a.Name and ID>a.ID)>0方法:delete a from #T a where ID<>(select top 1 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)
--3、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 nulldrop 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godelete a from #T a where exists(select 1 from #T where Num=a.Num and Name=a.Name and ID>a.ID)--只保留一条记录goalter table #T drop column ID--删除标识列--查看结果select * from #T
