Question

For the following (simplified) mysql DB setup, I’d like to copy the applicable guids into the message table. Can this be done with a single SQL update?

CREATE TABLE IF NOT EXISTS `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `guid` varchar(13) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;

INSERT INTO `user` (`id`, `guid`) VALUES
(1, 'a'),
(2, 'b'),
(3, 'c');


CREATE TABLE IF NOT EXISTS `message` (
  `user` int(11) NOT NULL,
  `user_guid` varchar(13) NOT NULL,
  KEY `user` (`user`)
) ENGINE=InnoDB;
ALTER TABLE `message`
  ADD CONSTRAINT `message_ibfk_1` FOREIGN KEY (`user`) REFERENCES `user` (`id`) ON DELETE CASCADE;

INSERT INTO `message` (`user`, `user_guid`) VALUES
(3, ''),
(2, ''),
(3, '');

Answer 1

Use:

UPDATE MESSAGE
   SET user_guid = (SELECT u.guid
                      FROM USER u
                     WHERE u.id = MESSAGE.user)
 WHERE EXISTS(SELECT NULL
                FROM USER u
               WHERE u.id = MESSAGE.user)