Question

我尝试获取最新创建的产品价格。每个产品都是独一无二的，但价格可能不同。但是，仅当产品的价格超过product_price表中的行时，我的查询才有效：

这是我的疑问：

SELECT 
    i.name AS title, 
    i.id AS product_id, 
    m.name AS manufacturer, 
    image, 
    price_sales, 
    price_new, 
    price_used, 
    price_old 
FROM product_info as i 
    LEFT JOIN product_manufacturer AS m ON i.manufacturer_id = m.id
    LEFT JOIN (SELECT * FROM product_price ORDER BY created_at DESC LIMIT 1) AS p ON i.id = p.id_product
WHERE category_id = 2 
    AND i.is_deactivated IS NULL 
LIMIT 0, 20;

我只需要最新创建的价格行。

Result

Answer 1

你遇到的问题是子查询：

(SELECT * FROM product_price ORDER BY created_at DESC LIMIT 1)

没有获得每件产品的最新价格，只是最新的价格，因此只返回一行，这意味着只有一件产品实际上会有价格。

解决此问题的方法是删除存在较新价格的任何价格，因此为简单起见，如果仅查看价格表，以下内容仅为您提供最新产品价格：

SELECT  p.*
FROM    product_price AS p
WHERE   NOT EXISTS
        (   SELECT  1
            FROM    product_price AS p2
            WHERE   p2.id_product = p.id_product
            AND     p2.created_at > p.created_at 
        );

然而，MySQL will optmise LEFT JOIN/IS NULL better than NOT EXISTS（虽然我认为前者表达的意图更好），所以更有效的方法是：

SELECT  p.*
FROM    product_price AS p
        LEFT JOIN product_price AS p2 
            ON p2.id_product = p.id_product
            AND p2.created_at > p.created_at 
WHERE   p2.id IS NULL;

最后，将此介绍回主要查询，最终会得到：

SELECT  i.name AS title, 
        i.id AS product_id, 
        m.name AS manufacturer, 
        i.image, 
        p.price_sales, 
        p.price_new, 
        p.price_used, 
        p.price_old 
FROM    product_info as i 
        LEFT JOIN product_manufacturer AS m 
            ON m.id = i.manufacturer_id
        LEFT JOIN product_price AS p 
            ON p.id_product = i.id
        LEFT JOIN product_price AS p2 
            ON p2.id_product = p.id_product
            AND p2.created_at > p.created_at 
WHERE   i.category_id = 2 
AND     i.is_deactivated IS NULL 
AND     p2.id IS NULL
LIMIT 0, 20;

如果表中只有一列，则LEFT JOIN返回NULL

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