如何在表中实现约束，使得插入的值不会仅对行组重复

CREATE TABLE `Error` (                                                                                                         
      `ErrorGID` int(10) unsigned NOT NULL auto_increment,                                                                         
      `Message` varchar(128) default NULL,                                                                                         
      `Created` timestamp NOT NULL default CURRENT_TIMESTAMP 
      on update CURRENT_TIMESTAMP,                                          
      PRIMARY KEY (`ErrorGID`),                                                                                                   
      UNIQUE KEY `MessageIndex` (`Message`))
      ENGINE=MEMORY 
      DEFAULT CHARSET=latin1 
      ROW_FORMAT=FIXED 
      COMMENT='The Fail() procedure writes to this table twice to force a constraint failure.';

DELIMITER $$
DROP PROCEDURE IF EXISTS `Fail`$$
CREATE PROCEDURE `Fail`(_Message VARCHAR(128))
BEGIN
  INSERT INTO Error (Message) VALUES (_Message);
  INSERT INTO Error (Message) VALUES (_Message);
END$$
DELIMITER ;

DELIMITER $$
create trigger mytable_check before insert on test.mytable for each row
begin
 if new.id_category in ('list','of','special','categories')
    and exists
      (select * from mytable
       where id_category=new.id_category
         and keywords=new.keywords) then
    call fail(concat('id_category,keywords must be unique when id_category is: ',new.id_category));
 end if;
end $$
DELIMITER ;