select * from (select rownum a,t.* from testtab t) where a=2;
select * from (select rownum a,t.* from testtab t) where a=3;
select * from (select rownum a,t.* from testtab t) where a=4;不能为：
select * from (select rownum,t.* from testtab t) where rownum=2;或
select * from testtab where rownum=2;返回多行记录：
select * from testtab where rownum<=10;返回某段记录：(如取记录表中4-10行)
select * from (select rownum no,testtab.* from testtab where rownum<=10) where no>=4;返回有条件且经过排序的某段记录：
select rownum num1,tt.* from
(select rownum num,t.* from
(select EcodeInfo.* from EcodeInfo where a=1 order by ecode desc) t) tt
where num>19 and rownum<20 以为oracle是先提取记录再排序的，而oracle的rownum是在提取记录就已经生成，它先于排序操作，所以必须使用子查询先排序。不能为：
select * from tsettab where rownum>10;返回最后一行记录:
select * from (select rownum a,t.* from testtab t) where a=(select count(*) from testtab);返回最后N行记录：
select * from (select rownum a,t.* from testtab t) where a=(select count(*)-N from testtab);