1)
SELECT b.BrandName FROM Auto a
LEFT JOIN Brand b ON b.BrandId = a.BrandId
GROUP BY a.BrandId ORDER BY AVG(a.PRICE) DESC
LIMIT 1;
2)
SELECT COUNT(a.AutoId) FROM Auto a
LEFT JOIN Brand b ON b.BrandId = a.BrandId
WHERE b.BrandCountry="DE";
3)
Не могу сообразить