Question

The question I’ve got here may look silly, but it drives me a bit crazy, as I can’t get how the MySQL join works.

I have 2 tables:

user;
+--------------------+--------------+------+-----+---------+-------+
| Field              | Type         | Null | Key | Default | Extra |
+--------------------+--------------+------+-----+---------+-------+
| id                 | int(11)      | NO   | PRI | NULL    |       |
| name               | varchar(128) | NO   |     | NULL    |       |
+--------------------+--------------+------+-----+---------+-------+

game;
+--------------------+--------------+------+-----+---------+-------+
| Field              | Type         | Null | Key | Default | Extra |
+--------------------+--------------+------+-----+---------+-------+
| id                 | int(11)      | NO   | PRI | NULL    |       |
| source_id*         | int(11)      | NO   |     | NULL    |       |
| target_id*         | int(11)      | NO   |     | NULL    |       |
+--------------------+--------------+------+-----+---------+-------+

game.source_id and game.target_id are constraints that reference the user.id.

Now I want to select everything from game table and I want to include in the result names of 2 users, since game has 2 users. And I want those columns to be named as target_name and source_name specifically.

Tried this query:

SELECT
    g.id,
    u.name
FROM
    game g
RIGHT JOIN
    user u
ON
    u.id = g.source_id 
OR
    u.id = g.target_id
WHERE
    g.source_id = 1
OR
    g.target_id = 1
;

But as you may have noticed, I don’t alias u.name as target_name or source_name because I don’t know how to do that and end result looks like this:

+----+------+
| id | name |
+----+------+
| 1  | bo   |
| 1  | al   |
| 2  | jo   |
| 2  | jay  |
+----+------+

As you can see, there’s a game appearing multiple times in the result.

Expected result:

+----+-------------+-------------+
| id | target_name | source_name |
+----+-------------+-------------+
| 1  | bo          | al          |
| 2  | jo          | jay         |
+----+-------------+-------------+

Thanks in advance.

Answer 1

you need to join the table user twice on table game in order to get the two names for the game. I have added COALESCE because it is useful for for showing descriptive value for NULL columns.If for instance there is no value for source or target, the value that will be shown is `-no value-.

SELECT  a.*,
        COALESCE(b.name,'-no source-') sourceName,
        COALESCE(c.name,'-no target-') targetName
FROM    game a
        LEFT JOIN `user` b
            on b.id = a.source_ID
        LEFT JOIN `user` c
            on on c.id = a.target_id
-- your condition here