题解 | 浙大不同难度题目的正确率

select 
    qd.difficult_level,
    round(sum(case when qpd.result = 'right' then 1 else 0 end)/count(qpd.question_id),4) as correct_rate
from user_profile up
left join question_practice_detail qpd on up.device_id = qpd.device_id
join question_detail qd on qpd.question_id = qd.question_id
where university = '浙江大学'
group by difficult_level
order by correct_rate asc

round (x,n) 保留x的n位数

sum是求和， count是统计个数