Duplicate Values in Database...

Hi,
 I want to stop users being able to register with a username which already exists in an access database, but i am having problems. Below is a script i have wrote but i keep receiving an error saying
Cannot implicitly convert type 'int' to 'bool'The code i have used is shown below, please help me. Thank You
protected void Button1_Click(object sender, EventArgs e)
    {
        int @Count;
        OleDbConnection conn;
        OleDbCommand cmd;

        string cmdString = "SELECT @Count=[mem_id] FROM [users] WHERE " +
      & ...