SELECT u.id FROM users u
WHERE  u.id not in (SELECT l.user_id FROM mlinks l 
) 

SELECT u.id FROM users u
WHERE NOT EXISTS (SELECT 1 FROM links l 
WHERE l.user_id = u.id )

CREATE TABLE `users` (
  `id` int(64) NOT NULL AUTO_INCREMENT,
  `registred` varchar(50) CHARACTER SET latin1 NOT NULL DEFAULT '',
  PRIMARY KEY (`id`),
) ENGINE=MyISAM AUTO_INCREMENT=53415 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

CREATE TABLE `links` (
  `id` int(32) NOT NULL AUTO_INCREMENT,
  `mpoll_id` int(32) NOT NULL,
  `link` varchar(256) COLLATE utf8_unicode_ci DEFAULT NULL,
  `user_id` int(64) DEFAULT NULL,
  `status` tinyint(3) DEFAULT '0',
  `user_ip` varchar(16) COLLATE utf8_unicode_ci DEFAULT NULL,
  `created` datetime DEFAULT NULL,
  `modified` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `user_id_ind` (`user_id`) USING HASH
) ENGINE=MyISAM AUTO_INCREMENT=543073 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;