make an ID in a mysql table auto_increment (after the fact)

For example, here's a table that has a primary key but is not AUTO_INCREMENT:

mysql> CREATE TABLE foo (
  id INT NOT NULL,
  PRIMARY KEY (id)
);
mysql> INSERT INTO foo VALUES (1), (2), (5);

You can MODIFY the column to redefine it with the AUTO_INCREMENT option:

mysql> ALTER TABLE foo MODIFY COLUMN id INT NOT NULL AUTO_INCREMENT;

Verify this has taken effect:

mysql> SHOW CREATE TABLE foo;

Outputs:

CREATE TABLE foo (
  `id` INT(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=6 DEFAULT CHARSET=latin1

Note that you have modified the column definition in place, without requiring creating a second column and dropping the original column. The PRIMARY KEY constraint is unaffected, and you don't need to mention in in the ALTER TABLE statement.

Next you can test that an insert generates a new value:

mysql> INSERT INTO foo () VALUES (); -- yes this is legal syntax
mysql> SELECT * FROM foo;

Outputs:

+----+
| id |
+----+
|  1 | 
|  2 | 
|  5 | 
|  6 | 
+----+
4 rows in set (0.00 sec)

I tested this on MySQL 5.0.51 on Mac OS X.

I also tested with ENGINE=InnoDB and a dependent table. Modifying the id column definition does not interrupt referential integrity.

---

To respond to the error 150 you mentioned in your comment, it's probably a conflict with the foreign key constraints. My apologies, after I tested it I thought it would work. Here are a couple of links that may help to diagnose the problem:

• What does mysql error 1025 (HY000): Error on rename of './foo' (errorno: 150) mean?
• http://www.simplicidade.org/notes/archives/2008/03/mysql_errno_150.html

None of the above worked for my table. I have a table with an unsigned integer as the primary key with values ranging from 0 to 31543. Currently there are over 19 thousand records. I had to modify the column to AUTO_INCREMENT (MODIFY COLUMN'id'INTEGER UNSIGNED NOT NULL AUTO_INCREMENT) and set the seed(AUTO_INCREMENT = 31544) in the same statement.

ALTER TABLE `'TableName'` MODIFY COLUMN `'id'` INTEGER UNSIGNED NOT NULL AUTO_INCREMENT, AUTO_INCREMENT = 31544;