"SELECT FROM table WHERE ".
  ( $form_username ? "username = $form_username " : '' ).
    ( $form_username && $form_password ? 'AND ' : '' ).
  ( $form_password ? "password = $form_password " : '' ).
    ( ($form_username || $form_password) && $form_name ? 'AND ' : '' ).
  ( $form_name ? "name = $form_name" : '' );