Oracle Pagination

rownum为虚拟列，不是实际物理列

select rownum,t.code from sys_config t; // 取得行数和code列

select rownum, t.code from sys_config t where rownum <=20 order by t.code; // 取得行数<=20，并按code排序，排序失败

select rownum, code from (select * from sys_config order by code) where rownum <=20; // 取得前20行成功

select rownum, code from (select * from sys_config order by code) where rownum>=1 and rownum <=20; // 取得1到20行成功

select rownum, code from (select * from sys_config order by code) where rownum>=2 and rownum <=20; // 取得2到20行失败

select rn,code from (select rownum as rn,code from (select * from sys_config order by code)) where rn >=2 and rn <=20; // 取得2到20行成功

最后一行为分页查询SQL语句，其他分页语句：
select * from (select rownum as rn, T.* from (select * from table_name) T where rownum<40) where rn>21; // 效率高些
select * from (select rownum as rn, T.* from (select * from table_name) T) where rn between 20 and 40; // 效率低些

参考连接文章：

http://database.51cto.com/art/201106/270156.htm

http://database.51cto.com/art/200904/118737.htm