题解 | #统计每个学校的答过题的用户的平均答题数#

#方法1 SELECT u.university,ROUND(COUNT(q.question_id)/COUNT(DISTINCT q.device_id),4) AS avg_answer_cnt FROM user_profile u,question_practice_detail q WHERE u.device_id=q.device_id GROUP BY university; #方法2 SELECT u.university,ROUND(COUNT(q.question_id)/COUNT(distinct(q.device_id)),4) avg_answer_cnt FROM question_practice_detail q LEFT JOIN user_profile u ON q.device_id=u.device_id #GROUP BY university