主要考察 case when 用法

select a.age_cut 
      ,count(a.device_id)
from (select device_id
           ,(case when age<25 then "25岁以下"
                  when age is null then "25岁以下"
             else "25岁及以上" 
             end) as age_cut
      from user_profile) a
group by a.age_cut