1、面试的时候常碰到一个 问题，就是让写一张表中有id和name 两个字段，查询出name重复的所有数据，现在列下：
select * from xi a where (a.username) in (select username from xi group by username having count(*) > 1)
2、查询出所有数据进行分组之后，和重复数据的重复次数的查询数据，先列下：
select count(username) as ‘重复次数’,username from xi group by username having count(*)>1 order by username desc
3、一下为 查看别人的 结果，现列下：查询及删除重复记录的方法大全
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 peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)
and rowid not in (select min(rowid) from people group by peopleId having count(peopleId )>1)
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)
(二)
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
(三)
方法一
复制代码
declare @max integer,@id integer
declare cur_rows cursor local for
select 主字段,count() from 表名 group by 主字段 having count() >;
open cur_rows fetch cur_rows into @id,@maxwhile @@fetch_status=0
begin
select @max = @max -1
set rowcount @max
delete from 表名 where 主字段 = @id
fetch cur_rows into @id,@maxend
close cur_rows
set rowcount 0
复制代码
方法二＂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,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）
(四)
查询重复
select * from tablename where id in (select id from tablenamegroup by idhaving count(id) > 1)
https://www.cnblogs.com/wangfuyou/p/6058169.html
