select * from (select p.*,rownum from prod.product_offer p)
where rownum = (select count(*)/2 from prod.product_offer);
这样为什么什么都查不出来
select * from (select p.*,rownum id from prod.product_offer p)
where id = (select count(*)/2 from prod.product_offer);
而在rownum加了一个别名后就可以查询出来,这是怎么回事,望高手解答