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 has a foreign key product_id .

So in addition to SELECT * FROM products; we are interested in getting the number of buys each product has.

Answer 1

Something along the lines of:

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

should do the trick.

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

Thats about all I can offer with the info you've posted. Table structure of both would help a ton if the above doesn't work.

Answer 3

Join

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

Subquery

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