select university , difficult_level, round(ifnull(count(q.question_id) / count(distinct u.id),0),4) avg_answer_cnt
from user_profile u
inner join question_practice_detail q
on u.device_id=q.device_id
inner join question_detail q1
on q.question_id = q1.question_id
group by university , difficult_level
from user_profile u
inner join question_practice_detail q
on u.device_id=q.device_id
inner join question_detail q1
on q.question_id = q1.question_id
group by university , difficult_level