需要幫助獲取類別，其中多個文章

Question

我想在PHP中開發知識庫系統。我試圖只返回有一個或多個帖子的類別。我有以下SQL代碼：

SELECT `category_id`, `category_name` FROM `categories` WHERE(SELECT 
`category_id` FROM `posts` HAVING COUNT(`posts`.`post_id`) >= 1) ORDER BY 
`category_id` ASC 

我嘗試它，我得到的所有類別，而不只是那些有超過1周後在其中。我不確定問題是什麼。請幫助

Answer 1

如果你想1個或多個帖子，你只需要categories和posts之間的簡單INNER JOIN。這將僅返回兩個表中存在category_id的行，這意味着該類別至少有1個帖子。

SELECT DISTINCT c.category_id, c.category_name 
FROM categories AS c 
JOIN posts AS p ON c.category_id = p.category_id 

如果你真的想要的類別，其中有超過1後，您可以使用GROUP BY和COUNT(*)。

SELECT c.category_id, c.category_name 
FROM categories AS c 
JOIN posts AS p ON c.category_id = p.category_id 
GROUP BY c.category_id 
HAVING COUNT(*) > 1 

Answer 2

你缺少在WHERE語句要使用的柱：

SELECT `category_id`, `category_name` FROM `categories` WHERE category_id in 
(SELECT `category_id` FROM `posts` HAVING COUNT(`posts`.`post_id`) >= 1) 
ORDER BY `category_id` ASC 

Answer 3

＃1：>1代替>=1

＃2：你算的所有帖子，不是每個類別的職位。無論是切換到相關子查詢或更好：

SELECT `category_id`, `category_name` 
FROM `categories` AS c 
JOIN 
(SELECT `category_id` 
    FROM `posts` 
    GROUP BY category_id 
    HAVING COUNT(`posts`.`post_id`) > 1 
) AS p 
ON c.category_id = p.category_id 
ORDER BY `category_id` ASC