Re: algebra equations for Reference and FD constraints

Brian Selzer wrote:
...
> Codd wasn't being sloppy. It does not logically mean that time values are
> involved: it means that the instantaneous state of the data bank at one
> arbitrary point in time can be a different collection of relations than that
> for the instantaneous state of the data bank at another arbitrary point in
> time.

In that case better to call them a time-varying collection.  ...
> ...
> Database updates are indeed a relational model concept even though neither
> the algebra nor the calculus are sufficient to express them..
> ...

Try and tell that to a computer. Go on, I dare you. When that doesn't work try and show us a formal definition of update that doesn't depend on set difference and union. Remember 'FORMAL', not Oracle or SQL wishy-washy prose.

> I don't agree with Date's characterization, nor with his conception that a
> database is a collection of relvars. There are several reasons, not the
> least of which is that its intended interpretation relies upon the false
> assumption that key values rigidly designate individuals in the universe of
> discourse, but most importantly it is that Date asserts that relational
> assignment is a primitve operation when it is clear that information is lost
> when UPDATE is translated into assignment. Using the primitives insert,
> update and delete, it is possible to describe completely--down to the
> attribute value--what is different between successive databases. ...

That's a load of bull, along with the stuff I snipped. Eg., Date hardly ever talks about keys except in the context of data structure/constraints or when he's making fun of ER models. You appear to have a pathological mania for expanding the vocabulary of every problem so far as to make it insoluble (even farther, don't ask me why). When normal people find it enough to talk of values, you want to include 'states', when it's legit to talk of 'primitive' algebraic ops, you abscond with the word and attach it to non-logical, interpretive motives. Airy-fairy but not productive. I'll bet you're a consultant. At least when Celko pops off here, we know his motive. Received on Wed Dec 31 2008 - 21:40:37 CET