题解 | #统计每个用户的平均刷题数#
统计每个用户的平均刷题数
http://www.nowcoder.com/practice/f4714f7529404679b7f8909c96299ac4
SELECT
t1.university,
t3.difficult_level,
COUNT(t2.question_id) / COUNT(DISTINCT(t2.device_id)) as avg_answer_cnt
from
user_profile as t1,
question_practice_detail as t2,
question_detail as t3
WHERE
t1.university = '山东大学'
and t1.device_id = t2.device_id
and t2.question_id = t3.question_id
GROUP BY
t3.difficult_level;
太实用了，感谢马丁老师分享压箱底的私藏哈哈哈