public static function executeSingleton0(string $query)
  {
    $result = self::query($query);
    $row    = $result->fetch_array(MYSQLI_NUM);
    $n      = $result->num_rows;
    $result->free();

    if (self::$mysqli->more_results()) self::$mysqli->next_result();

    if (!($n==0 || $n==1))
    {
      throw new ResultException('0 or 1', $n, $query);
    }

    return $row[0];
  }