select u.university,q.difficult_level,round(count(qd.question_id) / count(distinct qd.device_id), 4)'avg_answer_cnt' from user_profile u left join question_practice_detail qd on qd.device_id = u.device_id left join question_detail q on q.question_id = qd.question_id group by u.university,q.difficult_level having avg_answer_cnt > 0
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