User autoincrement ID for username
With MyISAM, there's a neat trick you can do:

CREATE TABLE IF NOT EXISTS test (
  id INT NOT NULL AUTO_INCREMENT,
  name varchar(20) NOT NULL,
  PRIMARY KEY (name, id)
) ENGINE=MyISAM 
  DEFAULT CHARSET=latin1 
  AUTO_INCREMENT=1 ;


With the above PRIMARY KEY, if all users have different name, they will be given id = 1. If another user comes and chooses name that already exists, he will get id=2, etc.



UPDATE

It seems I misread your question and you only need to have name = 'U243' for user with id=243. If that's the case, you don't have to store the name at all. Use this (myISAM or InnoDB, it will work for both):

CREATE TABLE IF NOT EXISTS test (
  id INT NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (id)
) ENGINE=MyISAM 
  DEFAULT CHARSET=latin1 
  AUTO_INCREMENT=1 ;


and then, to retrieve the id and name:

SELECT 
    id
  , 'U' || id  AS name
FROM
    test ;