```
CREATE TABLE IF NOT EXISTS `users` (
  `id` int(11) NOT NULL auto_increment,
  `username` varchar(50) NOT NULL,
  `status` varchar(20) NOT NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `username` (`username`)
)

```

Please explain me what is doing **UNIQUE KEY 'username' (username')** statement in the example above and why **('username')** is written once again?