题解 | #浙大不同难度题目的正确率#

解题思路： 

1.联接三表，筛选university为‘浙江大学’的表作为 date1；

2.正确率的理解：结果为right的求和/所有结果的次数；

3.从date1中查询题目要求字段，及难度、正确率； 

4.group by 难度，并按照正确率升序排列

代码为：

select date1.difficult_level,
sum(if(date1.result='right',1,0))/ count(date1.result) as correct_rate
from 
    (select u.device_id,q.question_id,q.result result,qd.difficult_level difficult_level
    from question_practice_detail q 
    join user_profile u on (q.device_id =u.device_id )
    join question_detail qd on (qd.question_id=q.question_id )
    where u.university='浙江大学') 
    as date1
group by date1.difficult_level
order by correct_rate