INSERT INTO if not exists SQL server

Question

I have a database structured as follows:

users

userid (Primary Key)
username

group

Answer 1

Basically you can do it like this:

IF NOT EXISTS (SELECT * FROM USER WHERE username = @username)
    INSERT INTO users (username) VALUES (@username)

But seriously, how you're going to know if user visited your website for the first time? You have to insert records in table user, when somebody register on your website, not login.