25.

查询各科成绩前三名的记录

CREATE TABLE ss_score
        SELECT 
           a.c_id, a.c_name, b.s_score
    FROM
        Course a
    LEFT JOIN
        Score b
    ON a.c_id = b.c_id
    ORDER BY a.c_id;
SELECT 
    t.*
FROM
    (SELECT 
           a.*,
           (SELECT COUNT(s_score) FROM ss_score b WHERE a.c_id = b.c_id AND b.s_score > a.s_score)+1 rk
   FROM
           ss_score a
   ORDER BY a.c_id,rk) t
WHERE t.rk IN (1,2,3)