如果要输出 1-3行按语文平均分降序、4-6行按数学平均分降序、7-9行按英文平均分降序，怎样修改呢

SELECT class.c_id,class.name,score.course,AVG(score.mark) AS avg_score

FROM class

JOIN student

ON class.c_id=student.c_id

JOIN score

ON student.s_id=score.s_id

WHERE class.descrip='重点班'

GROUP BY class.c_id,score.course

ORDER BY avg_score DESC;