注意：round四位小数、先where再group by
select up.university,
       qd.difficult_level,
       round(count(qpd.question_id)/count(distinct qpd.device_id),4) as avg_answer_cnt
from question_practice_detail qpd
left join user_profile up
using(device_id)
left join question_detail qd
using(question_id)
where up.university='山东大学'
group by qd.difficult_level;