题解 | #计算用户的平均次日留存率#

select
  round(count(date2)/count(date1), 4) as avg_ret 
from
  (select distinct a.device_id,
      a.date as date1,
      b.date as date2
    from
      question_practice_detail a
      left join question_practice_detail b 
      on a.device_id = b.device_id
   and datediff(b.date,a.date) = 1
  ) c

DATEDIFF 是一个日期函数，用于计算两个日期之间的差距，并以指定的时间单位返回结果datediff(b.date,a.date) = 1

或者DATE_ADD(q1.date, interval 1 day)