删除重复记录

-- 如何按字段删除重复记录

一张表里面以两个字段为唯一字段，当几条记录的这两个字段完全相同时，需要删除重复项，如下表
a b c d
1 2 3 4
1 5 3 5
1 2 7 9
以a、b为唯一字段，第一条和第三条的a、b完全相同，所以，需要删除第一条记录1 2 3 4 或者第三条记录1 2 7 9
即如下结果：
a b c d
1 2 3 4
1 5 3 5
或
a b c d
1 5 3 5
1 2 7 9



CREATE TABLE Tb1(id int , [ a ] varchar ( 255 ), [ b ] varchar ( 255 ), [ c ] varchar ( 255 ), [ d ] varchar ( 255 ))
INSERT Tb1(id, [ a ] , [ b ] , [ c ] , [ d ] )
           SELECT 1 , ' 1 ' , ' 2 ' , ' 3 ' , ' 4 '
UNION ALL   SELECT 2 , ' 1 ' , ' 5 ' , ' 3 ' , ' 5 '
UNION ALL   SELECT 3 , ' 1 ' , ' 2 ' , ' 7 ' , ' 9 '
UNION ALL   SELECT 4 , ' 1 ' , ' 4 ' , ' 7 ' , ' 6 '

delete Tb1 where [ id ] not in ( select max ( [ id ] ) from Tb1 group by a,b )
select * from tb1

drop table tb1

如果要同时删除第一和第三行
即如下结果：
a b c d
1 5 3 5

语句如下：

delete m from tb t
inner join
(
select a ,b
from tb
group by a , b
having count ( * ) > 1
)n
on m.a = n.a and m.b = n.b
或
delete * from tb as m,
(
select a ,b
from tb
group by a , b
having count ( * ) > 1
)n
where m.a = n.a and m.b = n.b








-- ----------------------------------------------------------------------------------
在几千条记录里,存在着些相同的记录,用SQL语句,删除掉重复记录
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 )

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
-- ----------------------------------------------------------------------------------------------
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
set rowcount 0

方法二
　　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）
select * from tablename where id in (
select id from tablename
group by id
having count (id) > 1 )