Selecting multiple table values with doubled joins - mysql

Question

I have a query which selects values from different tables via JOIN. But I think I have a problem now, because one table needs to join in again with a different name, but somehow it doesn't work.
An example based on a social network strucutre:
Table "users":
+--------+-----------+
| userid | username |
+--------------------|
| 1 | userOne |
| 2 | userTwo |
| 3 | userThree |
+--------+-----------+
Table "posts":
+--------+---------+-------------------------------+
| postid | userid | text |
+--------------------------------------------------|
| 102 | 1 | "Haha i'm User one" |
| 103 | 1 | "And User one is the best" |
| 104 | 3 | "I'm having fun with user two"|
+--------+---------+-------------------------------+
Table "usertags":
+--------+---------------+
| postid | tagged_userid |
+------------------------|
| 104 | 2 |
+--------+---------------+
This is my query:
SELECT posts.postid,
posts.userid,
posts.text,
users.username,
IFNULL(GROUP_CONCAT(DISTINCT usertags.tagged_userid SEPARATOR ','), NULL) as
taggedusers_id,
IFNULL(GROUP_CONCAT(DISTINCT taggedusers.fullname SEPARATOR ','), NULL) as
taggedusers_name,
FROM posts
JOIN users ON posts.userid = users.userid
LEFT JOIN usertags ON posts.postid = usertags.postid
LEFT JOIN users as taggedusers ON usertags.tagged_userid = users.userid
GROUP BY posts.postid
ORDER BY posts.postid DESC
And that's the result i get:
+--------+---------+---------------------------------------------------------------------+
| postid | userid | text | username | taggedusers_id |
+-----------------------------------------------------------------------|----------------|
| 102 | 1 | "Haha i'm User one" | userOne | NULL |
| 103 | 1 | "And User one is the best" | userOne | NULL |
| 104 | 3 | "I'm having fun with user two"| userThree | 2
+--------+---------+-------------------------------+--------------------+----------------+
The problem:
The column 'taggedusers_name' shows up, but it always shows NULL. I want it to show the usernames of the users which are tagged.
Like this, but in the whole, big output
+---------------+-------------------+
| taggeduser_id | taggeduser_name |
+-----------------------------------|
| 2 | userTwo |
| 2,3 | userTwo,userThree |
| NULL | NULL |
+---------------+-------------------+
So, how is this possible? Do I need to make a multiple SELECT statement? I tried that already, but I failed at this too :/
I'd be glad for help!

Answer 1

The issue is that you're referencing users again without the alias. Since users is already being implicitly INNER JOIN (see this question What is the default MySQL JOIN behaviour, INNER or OUTER?) so the taggedusers table will have to meet the conditions that the author is the same as the tagged user ID.
SELECT posts.postid,
posts.userid,
posts.text,
users.username,
IFNULL(GROUP_CONCAT(DISTINCT usertags.tagged_userid SEPARATOR ','), NULL) as
taggedusers_id,
IFNULL(GROUP_CONCAT(DISTINCT taggedusers.fullname SEPARATOR ','), NULL) as
taggedusers_name,
FROM
posts
JOIN
users
ON posts.userid = users.userid
LEFT JOIN
usertags
ON posts.postid = usertags.postid
LEFT JOIN
users as taggedusers
ON usertags.tagged_userid = taggedusers.userid -- this is (I assume) what you meant
-- ON usertags.tagged_userid = users.userid -- this is your problem
GROUP BY
posts.postid
ORDER BY
posts.postid DESC
One way to avoid this is to always alias tables; in this case you could have aliased users as 'author' or something of the like.
The reason you didn't have the same problem with the distinct IDs is that it was on a table that was joined correctly.