神即道 道法自然 如来

MySQL之——查询重复记录、删除重复记录方法大全

查找所有重复标题的记录:

select title,count(*) as count from user_table group by title having count>1;
SELECT * FROM t_info a WHERE ((SELECT COUNT(*) FROM t_info WHERE Title = a.Title) > 1) ORDER BY Title DESC

一、查找重复记录

1、查找全部重复记录
[mysql]SELECT * FROM t_info a WHERE ((SELECT COUNT(*) FROM t_info WHERE Title = a.Title) > 1) ORDER BY Title DESC[/mysql]
2、过滤重复记录(只显示一条)
[mysql]Select * From HZT Where ID In (Select Max(ID) From HZT Group By Title)
[/mysql]
注:此处显示ID最大一条记录

二、删除重复记录

1、删除全部重复记录(慎用)
[mysql]Delete 表 Where 重复字段 In (Select 重复字段 From 表 Group By 重复字段 Having Count(*)>1)
[/mysql]
2、保留一条(这个应该是大多数人所需要的 ^_^)
[mysql]Delete HZT Where ID Not In (Select Max(ID) From HZT Group By Title)
[/mysql]
注:此处保留ID最大一条记录

三、举例

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
[mysql]select * from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)
[/mysql]
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
[mysql]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)
[/mysql]
3、查找表中多余的重复记录(多个字段)
[mysql]select * from vitae a where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)
[/mysql]
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
[mysql]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)
[/mysql]
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
[mysql]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)[/mysql]

四、补充

有两个以上的重复记录,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
[mysql]select distinct * from tableName
[/mysql]
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
[mysql]select distinct * into #Tmp from tableName
drop table tableName
select * into tableName from #Tmp
drop table #Tmp[/mysql]
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
[mysql]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)[/mysql]

点赞

发表评论