# 3.
UPDATE `cars`
SET
    `condition` = 'C'
WHERE
    `condition` IN ('A' , 'B')
        AND (`mileage` >= 800000 OR `mileage` IS NULL)
        AND `year` <= 2010
        AND `make` != 'Mercedes-Benz';

# 4.
DELETE FROM `clients`
WHERE
    `id` NOT IN (SELECT
        `client_id`
    FROM
        `courses`);