分页查询两种方式结果却不同

Question

select * from emp
where rowid in
(select rid from
(select rownum rn ,rid from
(select rowid rid from emp ) where rownum where rn>4) order by deptno;

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select * from
(select t.*,rownum rn from
(select * from emp ) t where rownum )where rn > 4;

为什么两个查询的结果不同， 第一个的结果还排不了序？？是索引的原因么？？