分组topN

数据库表结构:sc

sid：学生id

cid：课程id

socre：分数

需求：查询每门课程分数最高的三个学生

select a.* from sc a where 3 > (select count(*) from sc where a.cid=cid and a.socre>score)

order by a.cid,a.score desc

分析：利用子查询建立一个笛卡尔积，对于a表的每一行都跟另外一个表的每一行匹配，如果count的值小于N说明该行的分数在top N之内

select a.* from sc a left join sc b on a.cid=b.cid and a.score>b.score group by a.sid,a.cid,a.score having count(*) <= 3 order by a.gid,a.score desc