javascript coding for connecting SQL server 2005 through javascript.

I want to connect SQL server 2005 Database through javascript.

My perpose is that i have one registartion form , in that one button(Check Availabily) .when user click on that button then script must check that the value entered in the textbox is exist in the database or not.

i am developing web application in java servlet,if you have another idea about it please guide me,i am waiting...