Oracle rownum 案例解说

Oracle rownum 案例解说

选择表中的某一行记录：(理解：rownum是Oracle系统顺序分配为从查询返回的行的编号)
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);

----------------
 select   *   from   adminrole   where   rownum<=4
  minus  
  select   *   from   adminrole   where   rownum<2>

 select * from
  (select rownum row_id ,b.* from (select a.* from sorttable a order by sortid)b) 
 where row_id between 5 and 9;