题解 | 浙大不同难度题目的正确率
select difficult_level,
(
sum(case when b.result = 'right' then 1 else 0 end)/count(difficult_level)
) as correct_rate
from question_detail a
inner join question_practice_detail b
on a.question_id = b.question_id
inner join user_profile c
on c.device_id = b.device_id
where c.university = '浙江大学'
group by difficult_level
order by correct_rate
sum分组统计
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