select up.device_id,up.university,
count(qpd.question_id) as question_cnt,
sum(if(qpd.result='right',1,0)) as right_question_cut
from user_profile as up

left join question_practice_detail as qpd
on up.device_id = qpd.device_id and month(qpd.date)=8 

where up.university ='复旦大学'
group by up.device_id

新get一个知识点，sum里面可以套用if，也可以用if函数的快速模式来写