oracle删除重复记录，只保留rowid最小的记录

初探oracle删除重复记录，只保留rowid最小的记录

 

如题，初探oracle删除重复记录，只保留rowid最小的记录（rowid可以反映数据插入到数据库中的顺序）

一、删除重复记录可以使用多种方法，如下只是介绍了两种方法(exist和in两种)。

1.首先创建一个测试表。

create table my_users(

    id number,

    username varchar2(20),

    sal number

)

2.插入测试数据

begin

    for i in  1..10 loop

        insert into  my_users values(i,'carl_zhang',i+10);

    end loop;

end;



begin

    for i in  1..10 loop

        insert into  my_users values(i,'carl_zhang',i+20);

    end loop;

end;



insert into my_users values(100,'carl',20.3);



commit;

3.查看重复记录

select rowid,rownum,a.* from my_users a

where 1=1

and exists(

    select 'exist' from my_users b

    where 1=1

    and a.id=b.id

    and a.username=b.username

    having count(*)>1    

)

order by rowid

4.查看重复数据中，rowid最大的记录（rowid可以反映数据插入到数据库中的顺序）

select rowid,rownum,a.* from my_users a

where 1=1

and exists(

    select 'exist' from my_users b

    where 1=1

    and a.id=b.id

    and a.username=b.username

   -- having count(*)>1

    having count(*)>1 and a.rowid=max(b.rowid)

)

order by rowid

5.删除重复数据，保留rowid最小的记录

delete  from my_users a

where 1=1

and exists(

    select 'exist' from my_users b

    where 1=1

    and a.id=b.id

    and a.username=b.username

   -- having count(*)>1

    having count(*)>1 and a.rowid=max(b.rowid)

)

二、以上方法是通过exist实现，相比in、not in更加的快速。

1.如下，查看重复记录。

select rowid,rownum,a.* from my_users a

where 1=1

and (a.id,a.username) in(

    select b.id,b.username from my_users b

    where 1=1  

    having count(*)>1

    group by  b.id,b.username    

)

order by rowid

2.查看重复数据中，rowid最大的记录

select rowid,rownum,a.* from my_users a

where 1=1

and (a.id,a.username,rowid) in(

    select b.id,b.username,max(rowid) from my_users b

    where 1=1  

    having count(*)>1

    group by  b.id,b.username    

)

order by rowid

3.删除重复数据，保留rowid最小的记录

delete from my_users a

where 1=1

and (a.id,a.username,rowid) in(

    select b.id,b.username,max(rowid) from my_users b

    where 1=1  

    having count(*)>1

    group by  b.id,b.username    

)

 

 

分类:  oracle

标签:  数据库