mysql - 按数量对订单进行排序，甚至显示数量为 0 的位置

Question

我编写了这段 SQL:它返回 11 月份之后销售的商品。不过，我想显示所有已售出的商品，并在该月未售出的商品旁边贴上 0。 希望得到一些指导:

SELECT ItemID, SUM(Quantity)
FROM orderitems
WHERE OrderNumber IN(
    SELECT OrderNumber
    FROM `order`
    WHERE OrderDate > "2015-11-31") GROUP BY ItemID;

Answer 1

试试这个:

SELECT ItemID, 
       COALESCE(SUM(CASE 
                      WHEN o2.OrderDate > '2015-11-31' 
                      THEN Quantity 
                    END), 0) AS Sum_Of_Quantity 
FROM orderitems AS o1
LEFT JOIN (SELECT OrderNumber, OrderDate
           FROM `order`
           WHERE OrderDate > '2015-11-31') AS o2
   ON o1.OrderNumber = o2.OrderNumber
GROUP BY ItemID;