#思路：三表连接，筛选浙大学生，用难度分组聚合再计算即可。注意，未答题的浙大学生不显示，所以要用第二张表左联第一张表。
select difficult_level, 
count(if(result='right',1,null))/count(result) a
from question_practice_detail 
left join user_profile using(device_id)
left join question_detail using(question_id)
where university='浙江大学'
group by difficult_level
order by a