INSERT INTO if not exists SQL server

Question

I have a database structured as follows:

users

userid (Primary Key)
username

group

Answer 1

Here is a (probably oversimplified) example that addresses your issue. Note that it is always best to use parameters to prevent injection attacks, especially when verifying users.

CREATE PROCEDURE AddUser
  @username varchar(20)
AS
  IF NOT EXISTS(SELECT username FROM users WHERE username=@username)
    INSERT INTO users(username) VALUES (@username)