SELECT COUNT(q2.device_id)/COUNT(q1.device_id) avg_ret 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 q2.date=DATE_ADD(q1.date,interval 1 day)

需求分析：次日残存率=昨日、今日都在的用户/今日用户。 连表查询

解决思路:掌握DATE_ADD函数，理解题意