题解 | #统计复旦用户8月练题情况#

select a.device_id,a.university,count(b.question_id) question_cnt,count(b.result = 'right' or null) right_question_cnt
from (select * from user_profile where university = '复旦大学') a
left join (select * from question_practice_detail where date like '%-08-%') b
on a.device_id = b.device_id
group by a.device_id,a.university

count(b.result = 'right' or null)不太清楚为什么要加上or null