Ah OK, thanks MayMusic.
Not sure I would call it a bug though. In SQL Server and many rdbms it is quite a normal scenario to allow NULLS on fields involved in relationships as a foreign key. In fact, I am very surprised this is not implemented by default in the DataPages of Caspio, and I would say the bug is more likely the other way around - the tables are behaving correctly, and the DataPages incorrectly. But I am not a Caspio expert, just someone from a db/programming background.