The "Available Cars" query could include an OUTER JOIN to the RentedCars table and then use that relationship to EXCLUDE from the AvailableList any car that is in the Currently-Rented-out data set.
You could also use an IsRented column, but it then becomes very important that that column is correctly updated at all times (i.e. if a car is RETURNED and the IsRented column does not get set to FALSE (software bug, whatever) then you have 100-shades-of-grief!! sorting that out.
The IsRented column will query faster than "Is this car in the Currently Rented-out Cars data set",and if/when you have a lot of data that may well be a killer. The IsRented column approach will scale, to larger size, MUCH better. The sub-query "Is this car in the Currently Rented-out Cars data set" is self correcting - just take the car OFF Rental and it is immediately available again.