Mysql удалить тригером первую запись

CREATE TABLE IF NOT EXISTS `webchat_lines` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `author` varchar(16) NOT NULL,
  `gravatar` varchar(32) NOT NULL,
  `text` varchar(255) NOT NULL,
  `ts` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  KEY `ts` (`ts`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8;

DELIMITER |
CREATE TRIGGER `update_test` AFTER INSERT ON `webchat_lines`
FOR EACH ROW BEGIN
  DECLARE updatecount INT;
  Set updatecount = ( SELECT count(*) FROM `webchat_lines` ;
  if updatecount=4
        then
          DELETE FROM `webchat_lines` WHERE id NOT IN (SELECT MIN(id) FROM `webchat_lines` );
        end if;
END;