---

SELECT
    car.marka AS `Марка`,
    car.model AS `Модель`,

    COUNT(dogovor.id_dogovor)
        AS `Количество договоров`,

    ROUND(
        COALESCE(
            SUM(
                car.cena * 0.001 *
                DATEDIFF(
                    dogovor.data_vozvrata,
                    dogovor.data_vidachi
                )
            ),
            0
        ),
        2
    ) AS `Суммарная стоимость договоров`,

    ROUND(
        COALESCE(
            AVG(
                DATEDIFF(
                    dogovor.data_vozvrata,
                    dogovor.data_vidachi
                )
            ),
            0
        ),
        2
    ) AS `Средняя длительность аренды`

FROM car

LEFT JOIN dogovor
    ON car.id_car = dogovor.id_car

GROUP BY
    car.id_car,
    car.marka,
    car.model

ORDER BY
    `Суммарная стоимость договоров` DESC;