mysql> CREATE TABLE test(id int not null primary key auto_increment, data int);
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO test(data) VALUES(29);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO test(data) VALUES(42);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM test WHERE id IS NULL;
+----+------+
| id | data |
+----+------+
|  2 |   42 |
+----+------+
1 row in set (0.00 sec)

mysql> SELECT * FROM test WHERE id IS NULL;
Empty set (0.00 sec)

mysql> SELECT * FROM test;
+----+------+
| id | data |
+----+------+
|  1 |   29 |
|  2 |   42 |
+----+------+
2 rows in set (0.00 sec)