MySQL查询及删除重复记录的方法

时间:2017-10-19
(一)
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
 
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)
 
(四)、查询重复
 
select * from tablename where id in (

select id from tablename

group by id

having count(id) > 1

)

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