mysql> select * from a;
+------+------+
| a_id | name |
+------+------+
|    1 | foo  |
|    2 | bar  |
|    4 | baz  |
+------+------+
3 rows in set (0.00 sec)

mysql> select * from b;
+------+--------------------+
| b_id | descr              |
+------+--------------------+
|    2 | descr for bar      |
|    3 | descr for whatever |
|    4 | descr for baz      |
+------+--------------------+
3 rows in set (0.01 sec)

mysql> select * from a INNER JOIN b on a.a_id=b.b_id;
+------+------+------+---------------+
| a_id | name | b_id | descr         |
+------+------+------+---------------+
|    2 | bar  |    2 | descr for bar |
|    4 | baz  |    4 | descr for baz |
+------+------+------+---------------+
2 rows in set (0.01 sec)

mysql> select * from a LEFT JOIN b on a.a_id=b.b_id;
+------+------+------+---------------+
| a_id | name | b_id | descr         |
+------+------+------+---------------+
|    1 | foo  | NULL | NULL          |
|    2 | bar  |    2 | descr for bar |
|    4 | baz  |    4 | descr for baz |
+------+------+------+---------------+
3 rows in set (0.00 sec)

mysql> select * from a RIGHT JOIN b on a.a_id=b.b_id;
+------+------+------+--------------------+
| a_id | name | b_id | descr              |
+------+------+------+--------------------+
|    2 | bar  |    2 | descr for bar      |
| NULL | NULL |    3 | descr for whatever |
|    4 | baz  |    4 | descr for baz      |
+------+------+------+--------------------+
3 rows in set (0.01 sec)

SELECT
    id, count(id) as anzahl, sum(bewertung) as 'bew', sum(bewertung) / count(id) as durchschnitt
FROM
    bewertung
GROUP BY
    id
ORDER BY
    durchschnitt desc
LIMIT 10

sum(bewertung) / count(id) as durchschnitt