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 dwhere 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