MySQL join three tables(one column or another based on condition) - mysql

Question

I have three tables:
users: I need the username column of the row whose id column matches the uid argument for the query
matches: I need the id of the row, and one of liked or liker columns as uid. Meaning, if the uid the query gets as argument is let's say 34 and the liker is 34, I get the liked column, which could be 789 or whatever.
pic_url: it has several rows for one user, but I need only the row where profile_pic = 1 (which is only gonna be one)
So far I've written this:
SELECT
matches.id,
IF(matches.liker = ${uid}, matches.liked, matches.liker) AS matches.uid,
users.username,
pic_urls.url AS profilePic
FROM matches
JOIN users
ON matches.uid = users.id
JOIN pic_urls
ON users.id = pic_urls.user_id
WHERE profile_pic = 1
I don't know where to put the condition to filter the rows in the pic_urls table, so I get only the one where the profile_pic column is 1 (there's only one with this)
The tables in question are:
mysql> describe matches;
+-------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+----------------+
| id | int unsigned | NO | PRI | NULL | auto_increment |
| liker | int unsigned | NO | | NULL | |
| liked | int unsigned | NO | | NULL | |
+-------+--------------+------+-----+---------+----------------+
mysql> describe users;
+-----------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+----------------+
| id | int unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(50) | NO | | NULL | |
| firstname | varchar(50) | NO | | NULL | |
...
mysql> describe pic_urls;
+-------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+----------------+
| id | int unsigned | NO | PRI | NULL | auto_increment |
| user_id | int unsigned | NO | | NULL | |
| url | varchar(255) | NO | | NULL | |
| profile_pic | tinyint(1) | YES | | 0 | |
+-------------+--------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)
I want:
-------+-----------+-----------+-------------------+
| id | uid | username | profpic |
+-------+-----------+-----------+------------------+
| 1 | 33 | bob | someurl |
| 5 | 22 | frank | someurl |
+-------+-----------+-----------+------------------+
Where id and uid are columns from the matches table, username from users table, and profpic from pic_urls.

Answer 1

This was the query I was after:
SELECT
matches.id,
users.id AS uid,
users.username,
pic_urls.url
FROM matches
JOIN users ON IF(matches.liker = ${uid}, matches.liked, matches.liker) = users.id
JOIN pic_urls ON IF(matches.liker = ${uid}, matches.liked, matches.liker) = pic_urls.user_id
WHERE profile_pic = 1