Re: object algebra

> > TDM is a more general model than RDM.
>
> Since the RDM is fully general, it seems a difficult task.

RDM isn't fully general. If it is, why does it need NULLs? RDM's fundamental design ensures NULLs will occur in some applications.

IMO, one simple way to judge which data model maybe more generic, is to count the occurance of NULLs. The model which utilizes the least NULLs is probably more general. The presence of NULLs is usually a red flag of some type of mismatch (chapter 20 of Date's "Intro to DB Systems"). TDM has no NULLs. In theory, RDM can eliminate all NULLs, but requires non-standard techniques that are impractical (ie generic modelling, where all the data is in one table with one column). In TDM, the techinque to model data that is "rectangularish" or otherwise is the same and doesn't result in NULLs.

Another more significant way to judge which data model is more generic, is to analyze degree of closure over basic operations (intersection, union, negation). Under RDM, closure requires meeting rather strict criteria (chapter 6) in comparision to criteria for closure in TDM. NULLs hinder closure which in turn hinders recursion.

According to Date's 6th Ed, "missing information is not fully understood", "no fully satisfactory solution is known", "incorporation into model is premature" but "Codd now regards NULLs as an integral part of RDM".

> Can you quantify that?

If we model various data examples, we should find less NULLs with TDM than with RDM. For example, modelling 10 persons, each with different properties. Or the following problem:

Allow user to create any hierarchy of things. Each thing in the hierarchy can be of different type/class. Each thing can have 1 to many parents in the hierarchy. For all possible combinations of 2 children in the hierarchy find the closest ancestor.

A solution to the above using TDM is shown at www.xdb1.com/Example/Ex075.asp
Note, that although the example shows a hierarchy of similar things and each thing has exactly 2 parents, the solution works with hierarchy of different kinds of things with different number of parents.

If someone can show a solution for the above problem using RDM, the genericness of the models will become clearer. Received on Mon Feb 23 2004 - 06:41:43 CET