一、查询某个字段重复
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select *
from User u
where u.user_name in (select u.user_name
from User u
group by u.user_name having count(*) > 1)
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二，删除表中某几个字段的重复
例：表中有条六条记录。 其中张三和王五 的记录有重复
TableA
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id customer PhoneNo
001 张三 777777
002 李四 444444
003 王五 555555
004 张三 777777
005 张三 777777
006 王五 555555
如何写一个sql语句将TableA变成如下
001 张三 777777
002 李四 444444
003 王五 555555
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测试环境
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create table TableA ( id varchar(3),customer varchar(5),PhoneNo varchar(6))
insert into TableA select '001','张三','777777'
union all select '002','李四','444444'
union all select '003','王五','555555'
union all select '004','张三','777777'
union all select '005','张三','777777'
union all select '006','王五','555555'
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结果
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delete TableA from TableA Twhere
exists(
select 1fromtablea where customer=T.customer and phoneno=T.phoneno
andid < tt.id
)
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总结
该方法适用于有一个字段为自增性，例如本例中的：id
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delete 表名 from 表名 as Twhere
exists(
select 1from表名 where 字段A=T.字段A and 字段B=T.字段B,(....)
and自增列 < T.自增列
)
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三，查询并删除重复记录的SQL语句
查询及删除重复记录的SQL语句
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
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select * from people
where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)
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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
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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)
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注:rowid为oracle自带不用该.....
3、查找表中多余的重复记录（多个字段）
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select * from vitae a
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)
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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
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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)
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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
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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)
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(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；
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Select Name,Count(*) From A Group By Name Having Count(*) > 1
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如果还查性别也相同大则如下:
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Select Name,sex,Count(*) From A Group By Name,sex Having Count(*) > 1
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(三)
方法一
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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(*) >； 1
open cur_rows
fetch cur_rows into @id,@max
while @@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,@max
end
close cur_rows
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set rowcount 0 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
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select distinct * into #Tmp from tableName
drop table tableName
select * into tableName from #Tmp
drop table #Tmp
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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
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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)
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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
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select * from tablename where id in (
select id from tablename
group by id
having count(id) > 1
)
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