Question

我有一个电子商务商店的数据库。我想知道每个类别的有效产品数量。

代码：

SELECT c.id_category, COUNT(cp.id_product) AS nproducts
FROM ps_category AS c
LEFT JOIN ps_category_product AS cp ON cp.id_category=c.id_category
LEFT JOIN ps_product AS p ON p.id_product=cp.id_product
WHERE p.active=1
GROUP BY c.id_category 
ORDER BY nproducts ASC

但是，它没有显示具有0个产品的类别。我想念什么？

Answer 1

删除WHERE p.active=1，对于没有产品的任何类别都将失败（导致查询不返回这些类别的行），并将该条件移至该ON的{{1}}子句中。这样，您仍将为这些类别获得一个LEFT JOIN行，这将使它们显示为具有0个产品。

NULL

请注意，要确保只计算有效产品，您需要计算SELECT c.id_category, COUNT(p.id_product) AS nproducts FROM ps_category AS c LEFT JOIN ps_category_product AS cp ON cp.id_category=c.id_category LEFT JOIN ps_product AS p ON p.id_product=cp.id_product AND p.active=1 GROUP BY c.id_category ORDER BY nproducts ASC，而不是p.id_product。

SQL查询以获取每个类别的产品数

1 个答案: