oracle 删除相同行

1、表明   t    列名   a

delete from t where rowid in(

select t.rowid from t,

(

select  a,max(rid) rid from 

(

select a,rowid rid from  t where a in

(select * from t group by(a) having count(a)>1)

) group by a

)t2

where t.a=t2.a and t.rowid != t2.rid

)