select
    age_group,
    count(age_group) as user_count
from(
        select 
            *,
            case when age<20 then '20以下'
                when age>=20 and age<=50 then '20-50'
                when age>50 then '50以上'
                else '未填写'
            end as age_group
        from
            customers_info
) a
group by
    age_group