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
)