Searching A Database Field For Null Value

Hi,

I am building a website in ASP.net C# for a university project, and would like to search a table (Member) for a field (UserName) using a session variable Session["sUserName"]. If that field is null, then I would like to insert that session variable into the field to start to create a new user. However, I am getting errors saying that I am using invalid expression terms. My code is;

//Create the Command, passing in the SQL statement and the ConnectionString queryString = "SELECT UserName FROM Member WHERE (UserName = @myUsername); ";

SqlCommand cmd = new SqlCommand(queryString, sqlConn);cmd.Parameters.Add(new SqlParameter("@myUsername", Convert.ToString(Session["sUserName"])));

//If UserName is null, display confirmation, else display errorif (UserName == null) ;

{UserNameCheckLabel.Text = "Username okay";

String queryString = "INSERT INTO Member (UserName) VALUES(@myUsername); ";SqlCommand cmd = new SqlCommand(queryString, sqlConn); cmd.Parameters.Add(new SqlParameter("@myUsername", Convert.ToString(Session["sUserName"])));

}else;

{UserNameCheckLabel.Text = "That username is in use";

}

I have a feeling I should be checking the database for the UserName, but I'm not sure whether to put this in the SELECT statement part or as a method... I would be most grateful for any advice!

Many thanks,

Chima