从两个表中仅选择满足第二个表中多个条件的行

Question

有3张表：

• 帖子
• 帖子标签
• 标签

是否可以仅选择具有 2 个或更多特定标签的帖子？

为了选择所有带有特定标签的帖子，我这样做

SELECT * FROM posts, tags WHERE posts.id=tags.id_post AND post_tags.id_tag=1

我试过这个查询，但它给了我 post_tags.id_tag=1 或 post_tags.id_tag=2 的帖子

SELECT * FROM posts, tags WHERE posts.id=post_tags.id_post AND post_tags.id_tag IN (1,2)

Answer 1

SELECT posts.*
FROM posts
JOIN tags ON posts.id = tags.id_post
WHERE tags.id IN(1, 2)
GROUP BY posts.id
HAVING COUNT(tags.id_post) >= 2

Answer 2

如果我理解正确的话——

SELECT postName,count(*) as times                -- you can add fields or omit the count(*)
FROM 
(select * from posts where id in (1,2) posts     -- you can replace the whole line with `posts`. this way gives you more room for conditions
inner join 
(select * from tags where id_post in (1,2)) tags -- same as @posts
on posts.id=tags.id_post 
group by postName                                -- if you added some fieldNames at the first line - add them here too..
having times>=2
                                                 -- AND tags.id=1 -- you can specify more conditions here