Re: object algebra

> Do you make some distinction between relational and RDM,
> and if so, what is RDM?

RDM (data expressed as collections of tuples contrained by the header, etc) is less general than relational [ie ((((ab)cd)e)f)(gh)]. Any time you add rules to a system, it becomes less general. RDM adds "rectangularish" rules to relational.

> Tables are not relations, and neither of them are "rectangular."

A relation (table) by definition is "rectangularish". Per Date, a relation consists of 1) a heading of a fixed set of attributes 2) a body consisting of tuples (rows), each containing of values related to the heading.

You don't see anything "rectangularish" in the above ???

Because the fundamental building block in RDM is "rectangularish", dealing with missing data, trees, and complex structure is more difficult and sometimes impractical, although not impossible, since you have to use more smaller blocks to get the desired shape or eliminate NULLs.

> You can display, for example, a 4-dimensional tessaract (hypercube)
> as a table if you like - that doesn't mean it's rectangular.

It is not that you cant represent 4-th dimensional, trees and complex data structures using the "rectangularish" building block provided by RDM, it's just more difficult and sometimes impractical.

> The general case is for data to fit into relations

Reality doesn't always provide data that fits "rectangularish" relations.
And when it doesn't, RDM needs NULLs, as Codd has correctly recognized.

> You can be sufficiently general through the attribute types
> (and subtypes!) that your relations are defined over.

Because the basic representational block in RDM is "rectangularish", to represent 10 persons each with different properties without NULLs, one has to resort to additional tables. The need to "subtype attributes" in this case is artifical and brought on by NULLs, which RDM creates, not reality. Received on Tue Feb 24 2004 - 21:30:09 CET