21.

查询不同老师所教不同课程平均分从高到低显示

SELECT
    a.t_name,
  b.c_name,
  AVG(c.s_score) avg_s
FROM
    Teacher a
LEFT JOIN
    Course b
ON a.t_id = b.t_id
LEFT JOIN
    Score c
ON b.c_id = c.c_id
GROUP BY a.t_name, b.c_name
ORDER BY avg_s DESC