Altering existing unique constraint - mysql

Question

I have a table that was defined like this:
CREATE TABLE `Message` (
`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
`user_id` integer NOT NULL,
`user_to` integer NOT NULL,
`top_num` integer NOT NULL,
`priority` smallint NOT NULL,
`error` varchar(120) NOT NULL,
UNIQUE (`user_id`, `user_to`, `top_num`)
);
Later, I added another column to it, msg_type, like this:
ALTER TABLE Message ADD COLUMN msg_type SMALLINT(6) NOT NULL DEFAULT 0;
However, I have come to realize that I need to change my original UNIQUE constraint to include msg_type. I tried running
ALTER TABLE Message
ADD UNIQUE INDEX (`user_id`, `user_to`, `top_num`, `msg_type`);
but INSERTs into my table still fail, and the error message indicates that that is because the old uniqueness constraint fails.
When I call describe Messages in mysql I see the following:
+-----------------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| user_id | int(11) | NO | MUL | NULL | |
| user_to | int(11) | NO | MUL | NULL | |
| top_num | int(11) | NO | MUL | NULL | |
| priority | smallint(6) | NO | | NULL | |
| error | varchar(120) | NO | | NULL | |
| msg_type | smallint(6) | NO | | 0 | |
+-----------------+----------------------+------+-----+---------+----------------+
which makes it seem like msg_type really isn't part of the constraint... How can I alter the constraint that the table was defined with, short of recreating the table?

Answer 1

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`;

Answer 2

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`);

Answer 3

This is because you haven't deleted the first unique constraint you have created. Right now, you have two unique constraints on your table.
To delete a unique constraint, have a look at this post
Dropping Unique constraint from MySQL table

Answer 4

This is because you are adding unique index. Please first drop unique index and then add unique constraint.
DROP INDEX index_name ON table_name
and now add unique constraint.
ALTER TABLE Message
ADD CONSTRAINT uc_message UNIQUE ((`user_id`, `user_to`, `top_num`, `msg_type`);)