执行顺序
Oracle会先设置行的rownum，再进行order by
执行顺序：rownum > order by
正确的写法应该是先order by，先包一层查询，再赋rownum值
select *
from (select rownum rno, t1.*
from (select *
from (select sysdate + level level_ from dual connect by level <= 10) t
order by level_ desc) t1) t2
where t2.rno <= 3