I understand that Gemstone is an Object-Oriented Database that stores objects and is great at that. But can you query those objects with SQL? What I'd like to

I understand that Gemstone is an Object-Oriented Database that stores objects and is great at that. But can you query those objects with SQL? What I'd like to understand is, is there some basic reason why RDBMS and ODBMs must be different databases. Or could their conceptual models of data perhaps be generalized into a single model. Using hyper-graphs perhaps. :-)