Re: Examples of SQL anomalies?

From: David Cressey <cressey73_at_verizon.net>
Date: Thu, 10 Jul 2008 12:18:01 GMT
Message-ID: <Z5ndk.1168$Ae3.265@trnddc05>

"Brian Selzer" <brian_at_selzer-software.com> wrote in message news:whadk.31863$ZE5.6401_at_nlpi061.nbdc.sbc.com...
>
> "David Cressey" <cressey73_at_verizon.net> wrote in message
> news:oe9dk.1109$Ae3.832_at_trnddc05...
> >
> > "Marshall" <marshall.spight_at_gmail.com> wrote in message
> > news:19040b82-0130-479e-ab80-dc1f1597ac02_at_56g2000hsm.googlegroups.com...
> >> On Jul 7, 1:21 pm, JOG <j..._at_cs.nott.ac.uk> wrote:
> >> > On Jul 5, 1:09 pm, "David Cressey" <cresse..._at_verizon.net> wrote:
> >> >
> >> > > "Marshall" <marshall.spi..._at_gmail.com> wrote in message
> >> >
> >> >
>news:6ad51b62-e66a-4daa-b21c-c361fd6b22f8_at_8g2000hse.googlegroups.com...
> >> >
> >> > > > What can be meaningfully asked is determined by the schema.
> >> >
> >> > > > If the schema specifies that the weight attribute is nullable,
> >> > > > then the question of how much a shipment weighs in total
> >> > > > is a question that cannot be asked.
> >> >
> >> > > > Marshall
> >> >
> >> > > Bingo!
> >> >
> >> > However, one /can/ validly ask "please give me the minimum the
> >> > shipment weighs" and this may still be very useful.
> >>
> >> Well, that's assuming that all the shipments have a positive
> >> weight. What if we might ship helium baloons? Then you
> >> can't set the minimum.
> >>
> >> OK I was just having some fun there.
> >>
> >>
> >> > This is of course
> >> > not a defence of null markers (obviously not. its me), but rather
just
> >> > a precaution against ruling out all questions of irregular data in
> >> > blanket fashion. Our aim should be to provide frameworks that allows
> >> > us to ask these questions with syntactic correctness /and/ as
> >> > parsimoniously as possible.
> >>
> >> Sure.
> >>
> >> My idea of the phrasing of the sum() over a nullable weight
> >> is "what is the sum of the weights of the items in the shipment
> >> for which the weight has been entered into the db?"
> >>
> >>
> > That is, IIUYC , the sum of all the wieghts, as far as the db knows
them,
> > right?
> >
> > This amounts to the "open world assumption" with regard to weights of
> > items.
> >
> > If one is going to adopt a "closed world assumption" with regard to
> > weights
> > of entered items, then it would seem to me that the weights column
would
> > have to be declared not nullable. Meaning that, if an item is entered
> > nto
> > a row, the weight column may not be left null.
> >
> > In the case of not nullable columns, this entire subthread is moot.
> >
> > Brian seems to be willing to bounce back and forth between OWA and CWA,
> > without making any entries in the schema to reflect which assumption is
> > operative. Or maybe it's just a disconnect between Brian and SQL.
>
> I don't think that's right.
>
> CWA: "If there ain't no row, then it ain't so."
>
> But there is a row:
>
> Package {{PackageNumber:12341, Weight:NULL}}
>
> Translation: there is a Package with a PackageNumber that is an element of
> the set of all of PackageNumbers and with a Weight that is an element of
the
> set of all Weights; oh, and by the way, the PackageNumber happens to be
> 12341.
>

I disagree. Your interpretation is that a package number with no recorded weight does, in fact, have a weight drawn for the set of all weights is an interpretation on your poart of the real world. It isn't inherent in the database schema.

There could be a database schema where a NULL in the wieght column is intended to convey that weight is inapplicable to this given package number. This is the same information that would be conveyed by omitting a row in a table that consists only of {PackageNumber, Weight}.

NULL means that there is no data here. It doesn't necessarily mean that the value is unknown. Received on Thu Jul 10 2008 - 07:18:01 CDT

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