I want to display count of children for every node in the tree.Дочерних, а не всех! Все приведённые решения неверные поскольку считают и сам узел.
-- Обновление минимальным кодом:
UPDATE Human
SET car_id = UniqCar.min_id
FROM Human
INNER JOIN Car ON Human.car_id = Car.car_id
INNER JOIN (SELECT MIN(car_id) AS min_id, model
FROM Car AS CarMini
GROUP BY model) AS UniqCar ON UniqCar.model = Car.model
-- Удаление лишних машин:
DELETE FROM Car
WHERE Car.car_id <> (SELECT MIN(CarOff.car_id) FROM Car AS CarOff WHERE CarOff.model = Car.model)