select count(q2.date)/count(q1.date)
from 
    (select distinct device_id,date 
    from question_practice_detail) q1
left join 
    (select distinct device_id,date 
    from question_practice_detail) q2
on q1.device_id=q2.device_id
and date_add(q1.date, interval 1 day)=q2.date