由于Oracle不支持select top N语句，所以在Oracle中需要利用order by和rownum的组合来实现select top N的查询。
rownum是记录表中数据编号的一个隐藏字段。
1.抽取top N数据
select id,name from (select id,name from customer order by name) a
where rownum <= N
order by rownum asc 2.在top N数据中抽出第M（M <= N ）条记录
select id,name from (select rownum as recno, id, name from (select id,name from customer order by name) awhere rownum <= Norder by rownum asc) b
where recno = M (M <= N) 3.抽取按某种方式排序的记录集中的第M条记录开始的X条记录
select id,name from (select rownum as recno, id, name from (select id,name from customer order by name) awhere rownum <= N --(N>=(M+X-1))order by rownum asc) b
where recno between M and (M+X-1) 在此基础上，可通过存储过程实现函数封装，将开始记录数以及抽取记录数作为输入参数，就可以轻松实现分页抽取数据。
END 2018-09-11 11:09:55
