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SQL如何按字段删除重复记录

--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 
  
 请问各位大侠这种sql语句怎么写 
  
  
 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,如何能用SQL语句,删除掉重复的呢?谢谢! 
 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)

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