Re: object algebra

> > Reality doesn't always provide data that fits "rectangularish" relations.
> > And when it doesn't, RDM needs NULLs, as Codd has correctly recognized.
>
> I also find odd the idea that "reality ... provide[s] data."

It was a figurative expression meaning reality can be the source for providing values in a relation's tuples. Reality may provide a person with two eyes and a person with no eyes. In RDM, the rules say define a relation's header at time X. RDM's rules forces us to presuppose that the attributes of future tuples will fit the definition at time X. This is rectangularish in nature. At time Y, we encounter Mary, and reality can't provide Mary's eyeColor so RDM create a NULL.

SS#, Name, EyeColor

1111, John, blue
2222, Mary, NULL
3333, NULL, black
4444, NULL, NULL
NULL, Luke, NULL
NULL, NULL, NULL

RDM creates the above "rectangularish" construct regardless of reality, thus incurring the penalty of NULLs. (This does not occur with TDM/XDb1) Received on Wed Feb 25 2004 - 23:24:39 CET