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

Question

I acquired a database from another developer. He didn\'t use auto_incrementers on any tables. They all have primary key ID\'s, but he did all the incrementing manually, in

Answer 1

ALTER TABLE `foo` MODIFY COLUMN `bar_id` INT NOT NULL AUTO_INCREMENT;

or

ALTER TABLE `foo` CHANGE `bar_id` `bar_id` INT UNSIGNED NOT NULL AUTO_INCREMENT;

But none of these will work if your bar_id is a foreign key in another table: you'll be getting

an error 1068: Multiple primary key defined

To solve this, temporary disable foreign key constraint checks by

set foreign_key_checks = 0;

and after running the statements above, enable them back again.

set foreign_key_checks = 1;