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

    a (or not)
    AND if a and b
...
    AND if (a or b or c) and d
    ...
    z (or not)

$sql_string = make_anded_statement ( username => $form_username,
                                     password => $form_password,
...
  }
  return "SELECT FROM table WHERE $where_string";
}