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

但仍然存在问题：其中没有产品的类别会返回itemCount = 1而不是0。我该如何解决这个问题？

Answer 1

你试过COUNT(products_to_categories.categoryID) AS itemCount吗？我不太确定，但会认为可能问题出在COUNT(*)。

Answer 2

尝试COUNT(product_name)或其他，而不是COUNT(*)。

Answer 3

COUNT(products_to_categories.categoryID)

要求COUNT(*)至少给你1，因为毕竟是 1行。具体计数需要特殊处理。

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'

SQL LEFT JOIN与COUNT（*）问题

4 个答案: