Re: algebra equations for Reference and FD constraints
Date: Mon, 29 Dec 2008 07:33:07 -0800
Brian Selzer wrote:
> is the case. That is the essence of database updates, which are definitely
> outside the scope of the algebra and the calculus.
> Database updates are indeed a relational model concept even though neither
> the algebra nor the calculus are sufficient to express them..
Taken together, those two sentences amount to a very familiar mysticism, trying to have it both ways. The RM is a model for a machine. Self-evident that it's not a model for people. For a db to be useful, obviously people must interpret the results of the model, ie., they agree to interpret its results the same way (in a given setting).
An example - obviously Codd expected that the practical purpose his audience had in mind was that some results would persist on some medium. But his model doesn't require that. Persistence is often mistaken as being essential to Codd's model, but it's not, the model stands up quite well without any notion of persistence. Do the same with any other concept, remove it from the picture and then ask is there a practical interpretation of the results of the calculus or algebra as applied to some formal definition of relation. Don't worry about whether everybody in the world prefers that interpretation, just ask whether some people do. As the posts from many people on various c.d.t. threads over the years show, there are many interpretations and yet very few concepts needed. This is became most concepts people talk about are interpretive concepts. Such concepts can be excised from all talk about the formal model without harm.
Same goes for 'updating'. If it is part of your interpretation of results, I don't care. If relvars are part of CJ Date's interpretation of results, I don't care about that either. I'm only interested in the logical model. In that, there are only different relation values and the symbolic manipulations a defined logic permits. Everything else is interpretation. To imply that those manipulations are based on some desired interpretation is to say that the interpretation is implementable in logic. Alright, so based on some of the prose that hits c.d.t, that means words like 'scope', et cetera, et cetera, must be expressed in logic. The history of language development outside of the db field, let alone within it, shows what a mug's game that is. Received on Mon Dec 29 2008 - 16:33:07 CET