24.

查询学生平均成绩及其名次

CREATE TABLE  avg_s_score
  SELECT 
    s_id, AVG(s_score) avg_s
  FROM 
    Score
  GROUP BY s_id
SELECT 
        s_id,
    a.avg_s,
    (SELECT COUNT(avg_s) FROM avg_s_score b WHERE b.avg_s > a.avg_s)+1 rk        
FROM
    avg_s_score a
ORDER BY avg_s