select

u.university,

d.difficult_level,

round(count(q.question_id)/count(distinct(q.device_id)),4) as avg_answer_cnt

from

user_profile as u,

question_practice_detail as q,

question_detail as d

where

u.device_id=q.device_id and

q.question_id=d.question_id

and u.university='山东大学'

group by d.difficult_level