select 临时表+group by+case when

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'
                when age is null then '未填写'
            end as age_group
        from
            customers_info
    ) as test
group by age_group