题解 | #统计复旦用户8月练题情况#

select up.device_id,up.university,count(q.device_id) question_cnt,sum(if(result = 'right',1,0)) right_question_cnt
from user_profile up
left join question_practice_detail q
on up.device_id=q.device_id 
where up.university='复旦大学' and (date like '2021-08-%' or date is null)
group by up.device_id;
这个题目有难度，如果直接用date是8月份去筛选的话，没有答题的同学会被忽略，下面这种解法也很好：
select up.device_id,up.university,count(q.device_id) question_cnt,sum(if(result = 'right',1,0)) right_question_cnt
from user_profile up
left join question_practice_detail q
on up.device_id=q.device_id and date like '2021-08-%'
where up.university='复旦大学'  
group by up.device_id;