mysql - SQL - 替换 HAVING COUNT(*) == 0

Question

所以我必须进行查询以返回所有不包含“apple”项目的收据号。

数据如下。 EG 如果您去商店购买苹果和香蕉，数据将是:

(table reciepts)
recieptNumber      productCode
12345              9999
12345              8888

(table products)
productCode        productName
9999               Apples
8888               Bananas

我在想:

SELECT reciepts.recieptNumber
FROM reciepts JOIN products
ON reciepts.productCode == products.productCode
WHERE products.productName == "Apples"
GROUP BY reciepts.recieptNumber
HAVING COUNT(*) == 0;

但我现在知道，在 count = 0 时，having 不起作用，因为没有什么可计算的。

有什么建议吗？

Answer 1

你的方法是在正确的轨道上:-)

SELECT reciepts.recieptNumber
FROM reciepts 
-- switch to Outer Join
LEFT JOIN products
ON reciepts.productCode = products.productCode
-- this will result in a NULL row when there's no Apple in the receipt
-- otherwise a row with 'Apples'
AND products.productName = 'Apples' 
GROUP BY reciepts.recieptNumber
-- if there's only a NULL you found the matching receipt 
HAVING COUNT(products.productCode) = 0;