MySQL的：選擇具有類別ID的所有帖子= 2和3

Question

我有以下的數據庫有2列：

posts id category id 
1   2 
2   1 
1   3 
3   3 
3   2 
4   5 

我需要一個查詢時提取所有帖子ID的是在第二和第三類 這意思是：id爲帖子：1和3

Answer 1

選擇所有帖子ID的是在2類和3：

SELECT posts_id 
FROM posts 
WHERE categoryId IN (2, 3) 
GROUP BY posts_id 
HAVING COUNT(distinct categoryId) = 2; 

來選擇關聯只只交的id，而不是任何其他：

SELECT posts_id 
FROM posts 
GROUP BY posts_id 
HAVING SUM(categoryId <> 5) = 0; 

Answer 2

SELECT post_id, category_id 
FROM posts 
WHERE category_id IN (2, 3) 
GROUP BY post_id 
ORDER BY category_id;