select up.university,qd.difficult_level,
round(count(qd.question_id)/count(distinct qpd.device_id),4)avg_answer_cnt
from (select * from user_profile where university = '山东大学') up
join
question_practice_detail qpd
using(device_id)
join
question_detail qd
using(question_id)
group by up.university,qd.difficult_level