Retrofit surrogate key in table with natural key in MySql?

Question

Assume a table that uses a natural key and has a number of existing rows. What would be the easiest way to retrofit a surrogate key column and populate it with unique values using MySql?

I.e. transform

table employees
(
    social_security_no varchar(20),
    ...
    constraint emp_pk primary key (social_security_no)
);

to

table employees
(
    id int,
    social_security_no varchar(20),
    ...
    constraint emp_pk primary key (id)
);

and fill the new id column with valid id:s.

Thanks /Erik

Answer 1

Simple:

ALTER TABLE employees DROP PRIMARY KEY;
ALTER TABLE employees ADD (id INT AUTO_INCREMENT PRIMARY KEY);

Optionally you'll (probably) want to do this:

CREATE INDEX employees_ssn ON employees (social_security_no);

and for any referencing tables:

ALTER TABLE blah ADD (employees_fk INT);
UPDATE blah b SET employees_fk = (SELECT id FROM employees WHERE social_security_no = b.ssn);
ALTER TABLE blah DROP (ssn);