select device_id,gender,
(
    case when age<20 then '20岁以下'
         when age<=24 then '20-24岁'
         when age>=25 then '25岁及以上'
         else '其他' end
) as age_cnt
from user_profile

CASE的用法：

(

case when xxx then xxx

when xxx then xxx

......

else xxx end

)