select up.university,
round(count(qpd.question_id)/count(distinct qpd.device_id),4) avg_answer_cnt
from user_profile up join question_practice_detail qpd on qpd.device_id=up.device_id
group by up.university  order by up.university

1、两张表联查。

2、平均数为：同一所大学的账号所答的题/同一所大学答题人数且结果保留4位小数。round(count(qpd.question_id)/count(distinct qpd.device_id),4)。

3、最后根据学校进行分组，然后排序。