Sequencing an Unsequenced Table

11.13.1 Problem

You forgot to include an AUTO_INCREMENT column when you created a table. Is it too late?

11.13.2 Solution

No, just add one using ALTER TABLE. MySQL will create the column and number the rows automatically.

11.13.3 Discussion

To add a sequence to a table that doesn't currently contain one, use ALTER TABLE to create an AUTO_INCREMENT column. Suppose you have a table t that contains name and age columns, but no sequence column:

+----------+------+ | name | age | +----------+------+ | boris | 47 | | clarence | 62 | | abner | 53 | +----------+------+

You can add a sequence column named id to the table as follows:

mysql> ALTER TABLE t -> ADD id INT NOT NULL AUTO_INCREMENT, -> ADD PRIMARY KEY (id); mysql> SELECT * FROM t ORDER BY id; +----------+------+----+ | name | age | id | +----------+------+----+ | boris | 47 | 1 | | clarence | 62 | 2 | | abner | 53 | 3 | +----------+------+----+

MySQL numbers the rows for you automatically. It's not necessary to assign the values yourself. Very handy.

By default, ALTER TABLE adds new columns to the end of the table. To place a column at a specific position, use FIRST or AFTER at the end of the ADD clause. The following ALTER TABLE statements are similar to the one just shown, but would place the id column first in the table or after the name column, respectively:

ALTER TABLE t ADD id INT NOT NULL AUTO_INCREMENT FIRST, ADD PRIMARY KEY (id); ALTER TABLE t ADD id INT NOT NULL AUTO_INCREMENT AFTER name, ADD PRIMARY KEY (id);

For MyISAM tables, you can specify the initial value for a new sequence column by including an AUTO_INCREMENT = n clause in the ALTER TABLE statement:

mysql> ALTER TABLE t -> ADD id INT NOT NULL AUTO_INCREMENT FIRST, -> ADD PRIMARY KEY (id), -> AUTO_INCREMENT = 100; mysql> SELECT * FROM t ORDER BY id; +-----+----------+------+ | id | name | age | +-----+----------+------+ | 100 | boris | 47 | | 101 | clarence | 62 | | 102 | abner | 53 | +-----+----------+------+