题解 | #统计每个学校的答过题的用户的平均答题数#
为什么一定要使用连接查询？？？
select university,count(question_id) / count(distinct question_practice_detail.device_id) avg_answer_cnt
from user_profile,question_practice_detail
where user_profile.device_id=question_practice_detail.device_id
group by university
不也一样？？
from user_profile,question_practice_detail
where user_profile.device_id=question_practice_detail.device_id
group by university
不也一样？？
