提取表中每个分组的前几条数据

SELECT  课程,
        SUM(奖金) AS 奖金
FROM    ( SELECT    课程,
                    学号,
                    成绩
          FROM      考试 AS a
          WHERE     ( SELECT    COUNT(*)
                      FROM      考试
                      WHERE     课程 = a.课程
                                AND 学号 <> a.学号
                                AND 成绩 > a.成绩
                    ) < 3
        ) AS b
GROUP BY 课程