问如何使mysql字段唯一？

EN

CREATE TABLE IF NOT EXISTS `hotaru_posts` (
  `post_id` int(20) NOT NULL AUTO_INCREMENT,
  `post_archived` enum('Y','N') COLLATE utf8_unicode_ci NOT NULL DEFAULT 'N',
  `post_updatedts` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `post_author` int(20) NOT NULL DEFAULT '0',
  `post_date` timestamp NULL DEFAULT NULL,
  `post_pub_date` timestamp NULL DEFAULT NULL,
  `post_status` varchar(32) COLLATE utf8_unicode_ci NOT NULL DEFAULT 'processing',
  `post_type` varchar(32) COLLATE utf8_unicode_ci DEFAULT NULL,
  `post_category` int(20) NOT NULL DEFAULT '1',
  `post_tags` text COLLATE utf8_unicode_ci,
  `post_title` text COLLATE utf8_unicode_ci,
  `post_orig_url` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `post_domain` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `post_url` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `post_content` text COLLATE utf8_unicode_ci,
  `post_votes_up` smallint(11) NOT NULL DEFAULT '0',
  `post_votes_down` smallint(11) NOT NULL DEFAULT '0',
  `post_comments` enum('open','closed') COLLATE utf8_unicode_ci NOT NULL DEFAULT 'open',
  `post_media` varchar(20) COLLATE utf8_unicode_ci NOT NULL DEFAULT 'text',
  `post_img` text COLLATE utf8_unicode_ci NOT NULL,
  `post_subscribe` tinyint(1) NOT NULL DEFAULT '0',
  `post_updateby` int(20) NOT NULL DEFAULT '0',
  `post_views` int(20) NOT NULL DEFAULT '0',
  `post_last_viewer_ip` varchar(32) COLLATE utf8_unicode_ci NOT NULL DEFAULT '111.111.111.111',
  PRIMARY KEY (`post_id`),
  KEY `post_archived` (`post_archived`),
  KEY `post_status` (`post_status`),
  KEY `post_type` (`post_type`),
  FULLTEXT KEY `post_title` (`post_title`,`post_domain`,`post_url`,`post_content`,`post_tags`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci COMMENT='Story Posts' AUTO_INCREMENT=38275 ;