select count(q2.device_id) / count(q1.device_id) from (select distinct device_id,date from question_practice_detail) as q1 left join (select distinct device_id,date from question_practice_detail) as q2 on q1.device_id=q2.device_id and date_add(q1.date,interval 1 day)=q2.date 1,主要是次日留存率的概念问题,留存率是指客...