题解 | #统计每个用户的平均刷题数#

select university,difficult_level,round(count(b.question_id)/count(distinct(b.device_id)),4) avg_answer_cnt
from user_profile a right join question_practice_detail b
on a.device_id = b.device_id
right join question_detail c
on b.question_id = c.question_id
group by university,difficult_level
having university = '山东大学'

总结错误：

在group by中没有加university字段导致报错

group by中出现的字段一定要出现在select中