Mysql 如果表中只有一列,则LEFT JOIN返回NULL
我尝试获取最新创建的产品价格。每种产品都是独一无二的,但价格可能会有所不同。但是,我的查询仅适用于产品在product_price表中具有多个price as行的情况: 这是我的疑问:Mysql 如果表中只有一列,则LEFT JOIN返回NULL,mysql,sql,greatest-n-per-group,Mysql,Sql,Greatest N Per Group,我尝试获取最新创建的产品价格。每种产品都是独一无二的,但价格可能会有所不同。但是,我的查询仅适用于产品在product_price表中具有多个price as行的情况: 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
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
您遇到的问题是子查询:
(SELECT * FROM product_price ORDER BY created_at DESC LIMIT 1)
不获取每个产品的最新价格,只获取最新价格,因此将只返回一行,这意味着您的产品中只有一个实际有价格
解决这一问题的方法是删除存在较新价格的任何价格,因此,为了简单起见,如果您只查看价格表,以下内容将仅为您提供最新的产品价格:
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,(尽管我认为前者更能传达意图),因此更有效的方法是:
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,将其引入主查询,您将得到:
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;