mysql> create table fun (i int);
Query OK, 0 rows affected (0.05 sec)

mysql> insert into fun values (1),(3),(null);
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from fun;
+------+
| i    |
+------+
|    1 |
|    3 |
| NULL |
+------+
3 rows in set (0.00 sec)

mysql> select avg(i) from fun;
+--------+
| avg(i) |
+--------+
| 2.0000 |
+--------+
1 row in set (0.00 sec)

mysql> select version();
+------------+
| version()  |
+------------+
| 4.1.20-log |
+------------+
1 row in set (0.00 sec)