关键点知识点多表合并及计算正确率(需要流程空值函数)

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