thanks for your reply. The Bicycle and the Motorbike are quite similar, whereas the Car has a completely different structure. That's why your suggestion with 3 columns (car_id, bicycle_id and motorbike_id) suits me best.
Thanks a lot!
I have a nother quick one: I have 2 tables continents (id, abbreviation) and countries (id, continent_id, name), where a continent has many countries.
With a given continent abbreviation, what's the fastest way to get the corresponding countries for that continent?