mysql> CREATE TABLE test ( testkey INT NOT NULL, testvalue INT );
Query OK, 0 rows affected (0.02 sec)

mysql> INSERT INTO test ( testkey, testvalue ) VALUES ( 1, 2 );
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO test ( testkey ) VALUES ( 3 );
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO test ( testvalue ) VALUES ( 4 );
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> SELECT * FROM test;
+---------+-----------+
| testkey | testvalue |
+---------+-----------+
|       1 |         2 |
|       3 |      NULL |
|       0 |         4 |
+---------+-----------+
3 rows in set (0.00 sec)

mysql>