一起使用 AVG 和 COUNT 在三个表之间进行 SQL 查询

Question

我遇到了这个超级复杂的 SQL 查询，我很想看看解决方案是什么。
所需要的是编写一个查询，该查询将返回所有客户多于平均数所有城市的客户。对于每个这样的城市，返回国家的名字，城市名称和客户数量.按国家名称排序结果上升.
以下是表格：

country: id, country_name  
city: id, city_name, postal_code, country_id  
customer: id, city_id, customer_name

我所能达到的最好结果是返回按城市分组的平均客户数。
有人可以输入查询吗？

Answer 1

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 
;