rownum 很早就会用，以前也经常用在分页查询，但今天发现了一个新问题：
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. 但是如果, 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，则可以解决，
select *
from (select rownum rn, t.*
from (select title, addtime from content order by addtime desc) t)
where rn >= 1
and rn <= 5
如果要考虑到效率的问题，上面的还可以优化成（主要两者区别）
select *
from (select rownum rn, t.*
from (select title, addtime from content order by addtime desc) t
where rownum <= 10)
where rn >= 3