select day(date) day, count(question_id) question_cnt from question_practice_detail where year(date) = 2021 and month(date) = 8 group by date

select (substring(date, 9)) day, count(question_id) question_cnt from question_practice_detail where date between '2021-08-01' and '2021-08-31' group by date

select device_id,gender,age,gpa from user_profile where university = '山东大学' union all select device_id,gender,age,gpa from user_profile where gender = 'male' 注意`union`是对结果去重的，所以要用`union all`

select t1.university, t3.difficult_level, (count(t2.question_id) / count(distinct t1.device_id)) avg_answer_cnt from user_profile t1 left join question_practice_detail t2 on t1.device_id = t2.device_id left join question_detail t3 on t2.question_id = t3.question_id where t1.u...

select t1.university, t2.difficult_level, (count(t2.question_id) / count(distinct t2.device_id)) avg_answer_cnt from user_profile t1, ( select t1.device_id, t2.question_id, t2.difficult_level from question_practice_detail t1, question_detail t2 where t1.question_id = t2.qu...