MySQL:返回带有 NULL 或 0 Vales 的计数 (*)

Question

我有三个表 CustomerSales 和 Icecream。为了便于理解，每次销售中都有一个冰淇淋。

CustomerSales 表包含(CustomerId、PurchaseId、邮政编码、IcecreamID)

Ice_Cream 包含(Ice_Cream 名称，Ice_Cream Id)

我正在尝试编写一个查询，将巧克力冰淇淋的销售额返回到每个邮政编码(邮政编码)，但我还想知道哪些邮政编码的销售额为零。目前它只返回有销售的邮政编码。

SELECT C.postcode, COUNT(*) AS TOTAL SALES
FROM CustomerSales C JOIN IceCream I
ON C.icecreamID = I.id AND
WHERE I.name = "Chocolate" AND C.saleyear = "2019"
GROUP BY C.postcode;

这与我想要的很接近，但只包括实现销售的邮政编码。我还想包括 2019 年巧克力冰淇淋销量为 0 的邮政编码。

我该怎么做？我尝试过汇总，但认为我做错了。

我也在想

SELECT C.postcode, COUNT(*) AS TOTAL SALES
FROM CustomerSales C OUTER LEFT JOIN IceCream I
ON C.icecreamID = I.id AND
WHERE I.name = "Chocolate" AND C.saleyear = "2019"
GROUP BY C.postcode;

我认为问题是，计数返回行的值。

Answer 1

计算 PurchaseId，而不是 *，我认为这应该返回预期的结果:

SELECT C.postcode, COUNT(C.PurchaseId) AS TOTAL SALES
FROM CustomerSales C OUTER LEFT JOIN IceCream I
ON C.icecreamID = I.id AND
WHERE I.name = "Chocolate" AND C.saleyear = "2019"
GROUP BY C.postcode;