SQL LEFT JOIN與COUNT（*）的問題

Question

我有以下查詢：

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 

但仍然有一個問題：類沒有產品在他們返回itemCount = 1而不是0。我怎樣才能解決這個問題？

Answer 1

你試過COUNT(products_to_categories.categoryID) AS itemCount？我不太確定，但會認爲問題可能在於COUNT(*)。

Answer 2

COUNT(products_to_categories.categoryID) 

問計COUNT(*)給你1至少因爲，畢竟還有爲 1排。具體計數需要特定的治療。

Answer 3

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' 

Answer 4

嘗試COUNT(product_name)或什麼的，而不是COUNT(*)。