public static function execute($conn, $sql) {
    $conn = is_array($conn) ? self::connect($conn) : $conn;
    $result = $conn->query($sql);
    if (!$result) {
      throw new SqlException("Cannot execute $sql: " . $conn->error);
    }

    if ($result && $result !== TRUE) {
      $result->free_result();
    }

  }