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,
    count(customer_id) AS user_count
FROM
    customers_info
GROUP BY
    age_group