Get all posts from a specific category - mysql

Question

The Situation
As some of you might already know from my previous questions, I'm currently developing a Blog-system.
This time, I'm stuck at getting all posts from a specific category, with their category.
Database
Here are the SQL-commands to create the three required tables.
Post
create table Post(
headline varchar(100),
date datetime,
content text,
author int unsigned,
public tinyint,
type int,
ID serial,
Primary Key (ID),
)ENGINE=INNODB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
author is the ID of the user who created the post, public determines if the post can be read from everyone or is just a draft and type determines if it's a blog-post (0) or something else.
Category
create table Kategorie(
name varchar(30),
short varchar(200),
ID serial,
Primary Key (name)
)ENGINE=INNODB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
Post_Kategorie
create table Post_Kategorie(
post_ID bigint unsigned,
kategorie_ID bigint unsigned,
Primary Key (post_ID, kategorie_ID),
Foreign Key (post_ID) references Post(ID),
Foreign Key (kategorie_ID) references Kategorie(ID)
)ENGINE=INNODB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
The Query
This is my current query to get all posts tagged with a specific category, which is determined by the category's ID:
SELECT Post.headline, Post.date, Post.ID,
CONCAT(
"[", GROUP_CONCAT('{"name":"',Kategorie.name,'","id":',Kategorie.ID,'}'), "]"
) as "categorys"
FROM Post
INNER JOIN Post_Kategorie
ON Post.ID = Post_Kategorie.post_ID
INNER JOIN Kategorie
ON Post_Kategorie.kategorie_ID = 2
WHERE Post.public = 1
AND Post.type = 0
GROUP BY Post.headline, Post.date
ORDER BY Post.date DESC
LIMIT 0, 20
The query works for listing all posts tagged with a specific category, but the categorys-column gets mixed up as every listed post has all available category's (every category listed in the Kategorie-table).
I'm sure the problem lays in the INNER JOIN-condition, but I have no clue where. Please point me in the right direction.

Answer 1

I suspect there might be issues with your CONCAT function, as it mixes different types of quotation marks. I think "[" and "]" should be respectively '[' and ']'.
Otherwise, the problem does seem to be with one of the joins. In particular, INNER JOIN Kategorie does not specify the joining condition, which, I think, should be Post_Kategorie.Kategorie_ID = Kategorie.ID.
There entire query should thus be something like this:
SELECT Post.headline, Post.date, Post.ID,
CONCAT(
"[", GROUP_CONCAT('{"name":"',Kategorie.name,'","id":',Kategorie.ID,'}'), "]"
) as "categorys"
FROM Post
INNER JOIN Post_Kategorie
ON Post.ID = Post_Kategorie.post_ID
INNER JOIN Kategorie
ON Post_Kategorie.Kategorie_ID = Kategorie.ID
WHERE Post.public = 1
AND Post.type = 0
GROUP BY Post.headline, Post.date
HAVING MAX(CASE Post_Kategorie.kategorie_ID WHEN 2 THEN 1 ELSE 0 END) = 1
ORDER BY Post.date DESC
LIMIT 0, 20
The Post_Kategorie.kategorie_ID = 2 condition has been modified to a CASE expression and moved to the HAVING clause, and it is used together with the MAX() aggregate function. This works as follows:
If a post is tagged with a tag or tags belonging to Kategorie.ID = 2, the CASE expression will return 1, and MAX will evaluate to 1 too. Consequently, all the group will be valid and remain in the output.
If no tag the post is tagged with belongs to the said category, the CASE expression will never evaluate to 1, nor will MAX. As a result, the entire group will be discarded.