Returns NULL if N is NULL.
mysql>
mysql> SELECT BIN(12);
+---------+
| BIN(12) |
+---------+
| 1100 |
+---------+
1 row in set (0.00 sec)
mysql>
mysql> SELECT BIN(NULL);
+-----------+
| BIN(NULL) |
+-----------+
| NULL |
+-----------+
1 row in set (0.00 sec)