如图上面是数据：
正确解法(假设分数一样则名次一样)
SELECT * FROM student t WHERE (SELECT COUNT(1)+1 FROM student WHERE class_id=t.class_id AND grade>t.grade)<=3
运行结果：
错误解法：来源于这里
SELECT * FROM student t WHERE EXISTS(
SELECT COUNT(*) FROM student WHERE grade>=t.grade AND class_id=t.class_id
GROUP BY class_id HAVING COUNT(*)<=3
) ORDER BY class_id,grade DESC;
运行结果：
有趣的是，上面的sql中，grade>=t.grade改为grade>t.grade运行结果好像是2,3,4名？？