Altering existing unique constraint

As in previous answer to change foreign key constraint use steps:

Step 1: Drop old constraint:

ALTER TABLE `Message` DROP INDEX `user_id`;

Step 2: Add new:

ALTER TABLE `Message` ADD UNIQUE INDEX (
         `user_id`, 
         `user_to`, 
         `top_num`, 
         `msg_type`);

---

Use SHOW CREATE TABLE to know name of constraint:

mysql> SHOW CREATE TABLE `Message` ;

| Message | CREATE TABLE `Message` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) NOT NULL,
  `user_to` int(11) NOT NULL,
  `top_num` int(11) NOT NULL,
  `priority` smallint(6) NOT NULL,
  `error` varchar(120) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `user_id` (`user_id`,`user_to`,`top_num`)
--           ^^^^^^^^^  name 
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

If you checks:

mysql> SHOW INDEX FROM `Message`;

Key_name is user_id that is first argument in UNIQUE (user_id ....

Suppose if you write:

ALTER TABLE `Message` ADD UNIQUE INDEX ( 
      `user_to`, 
      `user_id`, 
      `top_num`, 
      `msg_type`);

Then you have to drop using user_to as:

  ALTER TABLE `Message` DROP INDEX `user_to`;

This is because you are adding unique index. Please first drop unique index and then add unique constraint.

-- you have to drop each index one by one.
ALTER TABLE Message DROP UNIQUE INDEX user_id;

and now add unique constraint.

ALTER TABLE Message ADD CONSTRAINT uc_message UNIQUE (`user_id`, `user_to`, `top_num`, `msg_type`);