Re: object algebra

> > Could you give Date's reason/conditions/logic, why he thinks NULLs are
> > not integral part of RDM?
>
> Read the chapter, as I'm too tired right now to give more than this summary
> (and messages pile up, so I doubt I'll get back to this one): he talks about
> missing information.

I re-read Chapter 20 of 6th Ed. It does not indicate that NULLs aren't an integral part of RDM? Since he spends an entire chapter on the source of NULLs, the consequences of NULLs and how to deal with NULLs by replacing them with "disciplined default values", I would conclude that NULLs are an integral part of RDM and masking the problem with "disciplined default values" is the only practical/partial solution. If NULLs weren't an integral part of RDM, he could simply drop Chapter 20.

> Special values, on the other hand, avoid the problem by being
> specific values VALID WITHIN A DOMAIN (aka type). How you define them is up
> to you (ALSO DEPENDS ON THE TYPE). Therefore (drumroll please) their meaning
> is clear.

Given EyeColor = {red, brown, Gaping_Unblinking_Sockets} and the table Person EyeColor
------ --------------
John Gaping_Unblinking_Sockets
Mary Gaping_Unblinking_Sockets

Since John.EyeColor equals Mary.EyeColor, an AI program would incorrectly conclude that Mary and Bob have the same EyeColor. After incurring the initial NULL, masking them doesn't fix the problem entirely. Received on Sat Feb 28 2004 - 18:39:43 CET