查询及删除重复记录的方法

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)

先看表myemp

查出有重复数据的记录

查出没有重复数据的记录

查出不重复的记录

或者

select * from myemp e where rowid = (select max(rowid) from myemp e2 where e.userid = e2.userid and e.username = e2.username and e.salary = e2.salary)

如何删除重复数据

1、  当有大量重复数据存在并且在列userid,username,salary上有索引的情况下

delete myemp where rowid not in (select max(rowid) from myemp group by userid,username,salary);

2、 适用于少量重复数据的情况（当有大量数据时，效率很低）

delete myemp e where rowid <> (select max(rowid) from myemp e2 where e.userid = e2.userid and e.username = e2.username and e.salary = e2.salary);

3、 exception方法，适合大量重复数据的情况

首先建立exception表

然后添加约束，将错误记录到表exceptions中

建立重复数据临时表

删除有重复的所有数据

将临时表中的非重复数据重新插入原表