Re: Examples of SQL anomalies?

From: David Cressey <cressey73_at_verizon.net>
Date: Wed, 09 Jul 2008 20:31:16 GMT
Message-ID: <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.

> Marshall
Received on Wed Jul 09 2008 - 22:31:16 CEST