select count(distinct q.device_id,q.date) / (select count(distinct device_id,date) from question_practice_detail) as avg_ret from question_practice_detail q join question_practice_detail qt on q.device_id = qt.device_id and adddate(q.date, interval 1 day) = qt.date; 这里我们可以用最简单的方式:这里有一个隐藏的问题就是一个用...