database connection through js

I want some basic example of ajax in which dabase connection is used.I have serched I don't understand and not get.

I have used php,mysql

I have try for simple registration form in which 2 text field. one is username and 2 is password
When i enter the username it has to check that in database this user is exist or not without page submitting if php script is called.

Please tell me how i can know that from html in which js is included that this username is exist or not and if exist message will display.
Please reply me

re: database connection through js

I think no need to using ajax just using header function to pass the values another page and validate then return immediatly home hage.

No need to using ajax

vssp