select count(q2.device_id)/count(q1.device_id) as 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)

1.left Join: q1为今天的，q2的，用left就是今天的用户，明天还在； 2.date_add(time,interval )操作日期的