如何通过一个SQL查询获得产品以及每个产品的购买数量？

Question

Buy有一个外键product_id 。

因此除了SELECT * FROM products; 我们有兴趣获取每种产品的购买数量。

Answer 1

类似于以下内容：

SELECT p.*, COUNT(buy.product_id) FROM product p INNER JOIN buy ON buy.product_id = p.id GROUP BY buy.product_id

应该可以。

Answer 2

SELECT p.*,COUNT(b.product_id) FROM products p
INNER JOIN buy b ON p.id=b.product_id
GROUP BY b.product_id

那就是我所能提供的所有信息。 如果上述方法不起作用，两者的表结构都会帮助很多。

Answer 3

加入

SELECT Products.*, COUNT(Buy.ID) AS Bought
FROM products
    LEFT OUTER JOIN Buy ON products.ID = Buy.product_id
GROUP BY Buy.product_id

子查询

SELECT 
    Products.*, 
    (SELECT COUNT(ID) FROM Buy WHERE product_id = Products.ID) AS Bought
FROM products