select * from
(select * from student s where s.sex='男' order by s.score desc) where rownum<=3
union
select * from
(select * from student s where s.sex='女' order by s.score desc) where rownum<=3
然后这个UNION 与UNION ALL的区别:
UNION:操作符选取不同的值。
如果允许重复的值,就使用UNION aLL