Searching for results with an unknown amount of data

Hi,

I've got two tables, A and B, the first one containing columns

A_ID | info

where A_ID is primary, so that this table stores various information about 
an object, and the second containing columns

A_ID | property

where property is an integer referring to a particular property that an 
object may possess.  I've seperated these into two tables so that an object 
may have several (ie an unknown number of) properties.

Now, I want to find all objects that have at most properties 1,2,3, say (so 
something with (1,2) is okay, as is (1,2,3)). I can't see a way to do this 
-- can anyone help?
Also, what if I want to find all the objects possessing properties 4,5,6, as 
well as possibly other things? I've done it with nested SELECTs (constructed 
programmatically), but I think this is quite slow and not a very pleasing 
solution.

Obviously, both these things will need to be done for an arbitrary list of 
integers.

Thanks,
DL