How to set unique columns combination

Question

see this table:

    CREATE TABLE IF NOT EXISTS `default_messages_users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `message_id` int(11) NOT NULL,
  `owner_user_id` int(11) NOT NULL,
  `to_user_id` mediumint(8) NOT NULL
  PRIMARY KEY (`id`)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=1 ;

I need to set only one possible combination of the columns message_id, owner_user_id, to_user_id meaning the combination (for example) 1,1,1 only can exists one time so no duplicates is allowed. How I do this? Creating an index between them?

Cheers and thanks in advance

Answer 1

ALTER TABLE default_messages_users 
  ADD CONSTRAINT Unique__message__owner_user__to_user     --- you choose a sensible name
                                                          --- for the constraint
    UNIQUE message_id_owner_user_id_to_user_id_UQ         --- and the index 
      (message_id, owner_user_id, to_user_id) ;