建表和插入数据语句：

create table t
as
select object_name ename,
mod(object_id,50) deptno,
object_id sal
from all_objects
where rownum <= 1000

用相关子查询查出每个deptno里sal最高或者最低的3个人。

select *
  from t t1
where t1.sal in (select sal
                    from (select distinct sal, deptno
                            from t b
                          where b.deptno = t1.deptno
                          order by b.deptno, b.sal desc) a
                  where rownum <= 3)
但用

 select *
  from t t1
where t1.sal in (select sal
                    from (select distinct sal, deptno
                            from t b
                         
                          order by b.deptno, b.sal desc) a
                  where rownum <= 3 and b.deptno = t1.deptno )

确可以，请问这是什么原因？