MySQL 来自 FROM 的相关子查询

Question

我正在与 Sakila sample database 合作，并试图获得每个国家/地区观看次数最多的电影。到目前为止，我已经通过以下查询获得了某个国家/地区观看次数最多的电影：

SELECT 
    F.title, CO.country, count(F.film_id) as times
FROM 
    customer C 
INNER JOIN 
    address A ON C.address_id = A.address_id
INNER JOIN 
    city CI ON A.city_id = CI.city_id
INNER JOIN 
    country CO ON CI.country_id = CO.country_id
INNER JOIN 
    rental R ON C.customer_id = R.customer_id
INNER JOIN 
    inventory I ON R.inventory_id = I.inventory_id
INNER JOIN 
    film F ON I.film_id = F.film_id
WHERE 
    CO.country_id = 1
GROUP BY 
    F.film_id
ORDER BY 
    times DESC
LIMIT 1;

我想我将不得不在另一个查询的 FORM 中使用这个查询或类似的东西，但我已经尝试了所有我能想到的，但我完全无法弄清楚如何去做。

提前致谢！

Answer 1

我承认，这是一个非常难的查询。不过好吧，只要它有效。

解释：

子查询：与您已有的几乎相同。没有 WHERE 和 LIMIT。每个国家
结果，按国家/地区分组
GROUP_CONCAT(title ORDER BY times DESC SEPARATOR '|||')，将给出 'row' 中的所有标题，第一个是 most-viewed 标题。分隔符无关紧要，只要您确定它永远不会出现在标题中即可。
SUBSTRING_INDEX('...', '|||', 1) 导致字符串的第一部分，直到它找到 |||，在本例中是第一个（因此 most-viewed）标题

完整查询：

SELECT
    country_name,
    SUBSTRING_INDEX(
        GROUP_CONCAT(title ORDER BY times DESC SEPARATOR '|||'), 
        '|||', 1
    ) as title,
    MAX(times)
FROM (
    SELECT 
        F.title AS title, 
        CO.country_id AS country_id,
        CO.country AS country_name, 
        count(F.film_id) as times
    FROM customer C INNER JOIN address A ON C.address_id = A.address_id
    INNER JOIN city CI ON A.city_id = CI.city_id
    INNER JOIN country CO ON CI.country_id = CO.country_id
    INNER JOIN rental R ON C.customer_id = R.customer_id
    INNER JOIN inventory I ON R.inventory_id = I.inventory_id
    INNER JOIN film F ON I.film_id = F.film_id
    GROUP BY F.film_id, CO.country_id
) AS count_per_movie_per_country
GROUP BY country_id

概念验证（只要子查询正确）：SQLFiddle

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