请教个有点复杂的sql语句(内联视图)
数据表stud_db，字段yuwen
gridview上有A、B、C三列，我想让A显示字段yuwen的前20条记录，B列显示字段yuwen 21—40条记录，依此类推


select a.yumen as a,b.yumen as b,c.yumen as c from
(select  yuwen from
(select * from  stud_db order by xx)where rownum<=20) a,
(select  yuwen from
(select * from  stud_db order by xx)where rownum >=21 andrownum<=40) b,
(select  yuwen from
(select * from  stud_db order by xx)where rownum >=41 andrownum<=60) c

大虾帮忙详细解释下这条sql语句，谢谢！

select  yuwen from
(select * from  stud_db order by xx)where rownum<=20 按XX升序取出前20条yuwen字段的记录,
select  yuwen from
(select * from  stud_db order by xx)where rownum >=21 andrownum<=40 按xx升序取出21到40条yuwen字段的记录
select  yuwen from
(select * from  stud_db order by xx)where rownum >=41 andrownum<=60按xx升序取出41到60条yuwen字段的记录