SELECT f.flight_number, SUM((DATEDIFF(SECOND, 0, f.arrival_time) - DATEDIFF(MINUTE, 0, f.time_of_departure))) * COUNT(ofi.id) AS 'время'
    FROM flight_info AS f JOIN one_flight_info AS ofi ON f.flight_number = ofi.flight_id
GROUP BY f.flight_number

SELECT a.name AS 'компания-перевозчик',
    SUM((DATEDIFF(SECOND, 0, f.arrival_time) - DATEDIFF(MINUTE, 0, f.time_of_departure))) * COUNT(ofi.id) AS 'время'
    FROM airlines AS a JOIN flight_info AS f ON a.id=f.company_id
                            JOIN one_flight_info AS ofi ON f.flight_number = ofi.flight_id
GROUP BY a.name