php - 当左连接找不到匹配项时显示空结果

Question

我在加入 left 时遇到问题。它工作正常，但我没有看到空结果。它连接三个表——categories、product_categories 和 order_products。

堆栈用户发出的查询如下所示:

       SELECT
categories.name,categories.id,
SUM((orders_products.product_price_gross + orders_products.option_price)*(1 - (orders_products.rebate/100))  * orders_products.product_qty) as suma
FROM orders_products
LEFT JOIN product_categories
ON product_categories.product_id = orders_products.product_id
LEFT JOIN categories
ON product_categories.category_id = categories.id
GROUP BY categories.name, categories.id
ORDER BY suma DESC

我不确定如何升级此查询 - 我需要查看从未售出的商品以显示在商店统计信息中。

希望你能帮助我。

亲切的问候 标记

Answer 1

您想查看尚未售出的产品吗？您的主表是交易表orders_products。因此，任何未“订购”的产品都不会显示。互换 product_categories 和 orders_products 即可实现您想要的效果。如果 JOIN 语句找不到“order_products”表中列出的 product_id，这将列出所有类别并清空“suma”

SELECT
    categories.name,
    categories.id,
    SUM((orders_products.product_price_gross + orders_products.option_price)*(1 - (orders_products.rebate/100))  * orders_products.product_qty) AS suma
FROM product_categories
LEFT JOIN orders_products 
    ON product_categories.product_id = orders_products.product_id
LEFT JOIN categories
    ON product_categories.category_id = categories.id
GROUP BY categories.name, categories.id
ORDER BY suma DESC