看起来您需要嵌套的SELECTS和GROUP-BY：

SELECT
    *
FROM 
(
    /* compine first Inner-select with the second inner-select */
    SELECT 
        count(*) number_customer,
        city_name,
        country_name,
        avg_number_customer
    FROM 
        city,
        country,
        customer
        (
            /* Calculate average customer */
            SELECT 
                AVG(number_customer) avg_number_customer 
            FROM (
                /* GET number of customers for country/city */
                SELECT 
                    count(*) number_customer,
                    city_name,
                    country_name
                FROM 
                    city,
                    country,
                    customer 
                WHERE
                    city.id = customer_city_id
                    AND country.id = city_country_id
                GROUP BY
                    city_name,
                    country_name
            )
        )
    WHERE 
        city.id = customer_city_id
        AND country.id = city_country_id
    GROUP BY
        city_name,
        country_name,
        avg_number_customer
)
WHERE number_customer > avg_number_customer -- show only data, where number_customer > AVG 
;