oracle rownum对排序的影响

1. 标准的rownum分页查询使用方法：

    select *
      from (select c.*, rownum rn from content c)
     where rn >= 1
       and rn <= 5

    2. 可是若是, 加上order by addtime 排序则数据显示不正确

    select *
      from (select c.*, rownum rn from content c order by addtime)
     where rn >= 1
       and rn <= 5

    解决方法，再加一层查询，则能够解决，

    select *
      from (select rownum rn, t.*
              from (select title, addtime from content order by addtime desc) t)
     where rn >= 1
       and rn <= 5

    若是要考虑到效率的问题，上面的还能够优化成（主要二者区别）

    select *
      from (select rownum rn, t.*
              from (select title, addtime from content order by addtime desc) t
             where rownum <= 10)
     where rn >= 3