gistfile1.txt

mysql> SELECT AVG(rating), songs.song_id FROM songs LEFT JOIN ratings USING (song_id) GROUP BY songs.song_id;
+-------------+---------+
| AVG(rating) | song_id |
+-------------+---------+
|        NULL |       1 |
|      2.5000 |       2 |
|        NULL |       3 |
+-------------+---------+
3 rows in set (0.02 sec)

mysql> SELECT * FROM songs;
+---------+
| song_id |
+---------+
|       1 |
|       2 |
|       3 |
+---------+
3 rows in set (0.00 sec)

mysql> SELECT * FROM ratings;
+---------+--------+
| song_id | rating |
+---------+--------+
|       2 |      4 |
|       2 |      2 |
|       2 |      3 |
|       2 |      1 |
+---------+--------+
4 rows in set (0.00 sec)