初探oracle刪除重復記錄，只保留rowid最小的記錄

如題，初探oracle刪除重復記錄，只保留rowid最小的記錄（rowid可以反映數據插入到數據庫中的順序）

一、刪除重復記錄可以使用多種方法，如下只是介紹了兩種方法(exist和in兩種)。

1.首先創建一個測試表。

create table my_users(
    id number,
    username varchar2(20),
    sal number
)

2.插入測試數據

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.查看重復記錄

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.查看重復數據中，rowid最大的記錄（rowid可以反映數據插入到數據庫中的順序）

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.刪除重復數據，保留rowid最小的記錄

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、以上方法是通過exist實現，相比in、not in更加的快速。

1.如下，查看重復記錄。

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.查看重復數據中，rowid最大的記錄

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.刪除重復數據，保留rowid最小的記錄

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    
)