左联接找不到匹配项时显示空结果

Question

I have problem with join left. It work fine but i don't see null results. It connects three tables - categories, product_categories and order_products.

Query made by stack user look's like:

       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

I'm not sure how upgrade this query - I need to see what never been sold to show on shop statistics.

I hope You could help me.

Kind regards Mark

Answer 1

You want to see products that has not been sold? Your main table is the transaction table orders_products . So any product not "ordered" will not show. Interchange product_categories and orders_products to achieve what you want. This will list all categories and null out the "suma" if JOIN statement cannot find the product_id listed in the "order_products" table

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