Unique keys in MySQL

gistfile1.sql

mysql> show create table foo;
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                              |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| foo   | CREATE TABLE `foo` (
  `bar` varchar(128) DEFAULT NULL,
  `baz` varchar(128) DEFAULT NULL,
  UNIQUE KEY `foo_unique` (`bar`,`baz`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> insert into foo (bar, baz) values ('hello','world');
Query OK, 1 row affected (0.00 sec)

mysql> insert into foo (bar, baz) values ('hello','world');
ERROR 1062 (23000): Duplicate entry 'hello-world' for key 'foo_unique'
mysql> insert into foo (bar, baz) values (NULL, 'world');
Query OK, 1 row affected (0.01 sec)

mysql> insert into foo (bar, baz) values (NULL, 'world');
Query OK, 1 row affected (0.01 sec)