SELECT university,
    difficult_level,
    ROUND(COUNT(q.question_id)/COUNT(DISTINCT q.device_id),4) avg_answer_cnt
FROM user_profile u
JOIN question_practice_detail q
    ON u.device_id = q.device_id
JOIN question_detail qd
    ON q.question_id = qd.question_id
GROUP BY university,difficult_level
ORDER BY university,difficult_level