how to check is the username available?

[eluser]miauksius[/eluser]
Howdy,

When I was programing regular PHP, I was storing all data to txt files, and now, when I'm starting to use CI, I must use mysql. Can you please tell me, how to check username availability? That would help me to create the callback function for form validation.

THANK YOU

P.S. Sorry for the broken english