select 
    u.university,
    t.difficult_level,
    count(d.question_id)/count(distinct d.device_id)
from 
    user_profile as u,
    question_practice_detail as d,
    question_detail as t
where u.university = '山东大学' and
    u.device_id =d.device_id and
    d.question_id =t.question_id

group by t.difficult_level