题解 | #统计每个学校的答过题的用户的平均答题数#

1. 通过question_practice_detail表左连接user_profile表
2. 通过university分组
3. 聚合函数count计算出：各学校的答题总数count(device_id)/各学校的不同学生总数count(distinct device_id) 下面是完整的SQL：

select university,count(device_id)/count(distinct device_id) as avg_answer_cnt from (select question_practice_detail.device_id,university from question_practice_detail LEFT JOIN user_profile ON question_practice_detail.device_id = user_profile.device_id ) as d group by university