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select

    university,

    difficult_level,

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

from question_practice_detail as qpd

left join user_profile as up

on up.device_id=qpd.device_id

left join question_detail as qd

on qd.question_id=qpd.question_id

group by university, difficult_level