SELECT * FROM course a WHERE 3>(SELECT COUNT(*) FROM course WHERE coursename=a.coursename and score>a.score) ORDER BY a.coursename,a.score DESC;取每一条记录，判断同一个班级，大于当前成绩的同学是不是小于2个人。
mysql中分组之后取每个组的前三个
原文：https://www.cnblogs.com/fenghua/p/9109160.html
