CREATE TABLE `users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `reference` int(11) DEFAULT NULL,
  `username` varchar(255) DEFAULT NULL,
  `group` varchar(255) DEFAULT NULL,
  `created_at` datetime DEFAULT NULL,
  `comment` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `index_users_on_reference` (`reference`),
  KEY `index_users_on_username_and_created_at` (`username`,`created_at`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8