sql - SQL LEFT JOIN with COUNT(*) 问题

Question

我有以下查询：

SELECT products_categories.categoryID, name, COUNT(*) AS itemCount
FROM products_categories
LEFT JOIN products_to_categories ON products_to_categories.categoryID = products_categories.categoryID
GROUP BY products_categories.categoryID

但是仍然存在一个问题：其中没有产品的类别返回itemCount = 1而不是0. 我怎样才能解决这个问题？

Answer 1

你试过COUNT(products_to_categories.categoryID) AS itemCount吗？我不太确定，但会认为问题可能出在COUNT(*).

Answer 2

尝试COUNT(product_name)或其他方式，而不是COUNT(*).

Answer 3

COUNT(products_to_categories.categoryID)

要求COUNT(*)至少给你 1，因为毕竟有1行。具体计数需要具体处理。

Answer 4

SELECT products_categories.categoryID, name, COUNT(*) AS itemCount
FROM products_categories
LEFT JOIN products_to_categories ON products_to_categories.categoryID = products_categories.categoryID
GROUP BY products_categories.categoryID
WHERE itemCount <> '0'