题解 | #统计每个用户的平均刷题数#

SELECT `university`,`difficult_level`,ROUND(COUNT(qpd.question_id)/COUNT(DISTINCT(qpd.device_id)),4) AS `avg_answer_cnt`
FROM `user_profile` AS `up`
JOIN `question_practice_detail` AS `qpd` ON `up`.`device_id`=`qpd`.`device_id`
JOIN `question_detail` AS `qd` ON `qpd`.`question_id`=`qd`.`question_id`
WHERE `university`='山东大学'
GROUP BY `difficult_level`;