题解 | #统计复旦用户8月练题情况#

select up.device_id,
       university,
       count(question_id) as question_cnt,
       sum(if(result = 'right',1,0))as right_question_cnt
from user_profile up
left join question_practice_detail qpd
    using(device_id)
where university = '复旦大学'
      and (month(date) = 8
      or month(date) is null)
group by up.device_id, university

注意点：

1. 正确题目计数，不用count，用sum和if组合
2. 因有未答题同学，left join保险
3. where筛选月份时，注意增加null条件，避免漏掉未答题同学，用or连接