MySql:选择类别为id=2和3的所有帖子

Question

我有以下包含2列的数据库：

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

我需要一个查询，当提取所有的帖子id是在类别2和3，这意味着:帖子id: 1和3。

Answer 1

要选择类别2和3中的所有帖子id：

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

要仅选择仅与category_id = 5关联而不与任何其他关联的帖子id：

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;