I am currently developing a CMS for a school project and I wanted to add the function to check, real-time, if a username is already taken. It should be a little bit like what happens when you register for a Gmail or Yahoo! account.
Basically, while the user inputs the desired username in a textbox, I want to make appear next to it a green or red sign (plus a small text) to display availability.
The registered usernames will be in a mysql table which I can query with a very basic SELECT statement.
It should work AS THE USER TYPES, so if he's entering "mrbrown" then it should try to query for every letter.
This will be my first AJAX implementation, so I am asking for a starting point and any suggestion you deem could help me.