（1）用if函数的写法。

SELECT IF(age>=25,"25岁及以上","25岁以下") AS age_cut,count(*) AS number
FROM user_profile
GROUP BY age_cut;

（2）case的写法。

select 
    (case
        when age>=25 then '25岁及以上'
        else '25岁以下' end) as age_cut, 
    count(*) as number
from user_profile
group by age_cut

两个as,还有记得count是GROUP BY之后计数的。