题解 | #统计复旦用户8月练题情况#

select 
    difficult_level, 
    sum(if(result='right', 1, 0)) / count(result) as correct_rate
from 
    (
        select 
        * 
        from user_profile
        where university='浙江大学') a
inner join question_practice_detail b on a.device_id=b.device_id
inner join question_detail c on b.question_id = c.question_id
group by difficult_level
order by correct_rate