Re: What predicates the following relation represents
Date: Sat, 03 Apr 2004 09:44:22 GMT
It has been postulated that there is a difference between an external and internal predicate (see Date). We can also gather that multiple external predicates can be evaluated to be internally logically equivalent as a predicate, but the opposite does not hold true (much like the view update problem).
Given this, if you don't know either the wff of a virtual or the external predicate (the constant part of the intension) of the base relation, then trying to deduce the true total external predicate is the equivalent of either presenting an unsound, unprovable, or at least an invalid argument.
Asserting that because the extension of a relation does not conform to what one thinks the external predicate is is a both a fallacy or at the very least a false conclusion - assuming the designers followed some semblance of logic in design and that at least entity integrity is enforced by the DBMS.
This is what makes data integration so difficult. Semantics (and yes concepts in the eye of the beholder) do matter.
"--CELKO--" <joe.celko_at_northface.edu> wrote in message
> >> It depends on the meaning given to the relation. <<
> That is what I am trying to pin down with the idea of a "complete
> fact" ..
> >> "At some point in time, we have sold a batch of PART of size SOLD."
> then I need that time to have my complete fact and I should add a
> timestamp column to the table.
> >> "Have we ever sold a batch of 7 nuts?"; "Show me all the parts we
> have ever sold as a batch of 10" <<
> then I need something to mark that batch to have my complete fact. If
> what we are calling "sales" in this example ought to be "batch_size",
> then I have it. And my key would be both columns, as you said.
> I always loved the informal idea of a "False Fact" versus a "True
> Fact" in practical reasoning. The "False Fact" statement "The pilot
> is sober today, so we will take off now" carries a lot of
> implications, even tho it is true. The complete facts might be that
> this pilot has a perfect flight history and has never had a drink in
> his life.
Received on Sat Apr 03 2004 - 11:44:22 CEST