code: def read_data_as_dataframe(self,
                                 travel_impedance_measure,
                                 from_stop_I=None,
                                 to_stop_I=None,
                                 statistic=None):
          
          to_select = []
          where_clauses = []
          to_select.append("from_stop_I")
          to_select.append("to_stop_I")
          if from_stop_I is not None:
              where_clauses.append("from_stop_I=" + str(int(from_stop_I)))
          if to_stop_I is not None:
              where_clauses.append("to_stop_I=" + str(int(to_stop_I)))
          where_clause = ""
          if len(where_clauses) > 0:
              where_clause = " WHERE " + " AND ".join(where_clauses)
          if not statistic:
              to_select.extend(["min", "mean", "median", "max"])
          else:
              to_select.append(statistic)
          to_select_clause = ",".join(to_select)
          if not to_select_clause:
              to_select_clause = "*"
          sql = "SELECT " + to_select_clause + " FROM " + travel_impedance_measure + where_clause + ";"
          df = pd.read_sql(sql, self.conn)
          return df
docs: Recover pre-computed travel_impedance between od-pairs from the database.
  
          Returns
          -------
          values: number | Pandas DataFrame