为什么 select AA3.difficult_level, sum(case when AA2.result='right' then 1 else 0 end)/count(AA2.question_id) as correct_rate from question_practice_detail AA2 left join user_profile as AA1 on AA1.device_id=AA2.device_id left join question_detail AA3 on AA3.question_id=AA2.question_id WHERE AA1.university='浙江大学' group by AA3.difficult_level order by correct_rate 和 select AA3.difficult_level, sum(case when AA2.result='right' then 1 else 0 end)/count(AA2.question_id) as correct_rate from question_practice_detail AA2 left join (select * from user_profile where university='浙江大学') as AA1 on AA1.device_id=AA2.device_id left join question_detail AA3 on AA3.question_id=AA2.question_id group by AA3.difficult_level order by correct_rate 输出的值不一样呢