select user_id,room_id,room_type,days 
from(
select c.user_id,c.room_id,g.room_type,
datediff(c.checkout_time ,c.checkin_time) as days
from guestroom_tb g
join checkin_tb c on g.room_id = c.room_id)a
where a.days>1
order by days,room_id,user_id desc

记住datediff(c.checkout_time ,c.checkin_time)