Re: MV Keys
Date: Tue, 7 Mar 2006 11:53:25 +0200
Message-ID: <dujl6n$lg5$1_at_emma.aioe.org>
"Jon Heggland" <heggland_at_idi.ntnu.no> wrote in message
news:MPG.1e776ecdfa8ce13e989794_at_news.ntnu.no...
> In article <1141720100.794765.5770_at_e56g2000cwe.googlegroups.com>,
> marshall.spight_at_gmail.com says...
> > Jon Heggland wrote:
> > > marshall.spight_at_gmail.com says...
> > > >
> > > > But isn't a tuple a subtype of a relation?
> > >
> > > Not according to the definitions I prefer. I feel uncomfortable with
the
> > > circularity this would entail: That a relation is a set of
relations(?).
> >
> > Relation: a subset of a product of sets
> > Tuple: a subset of cardinality 1 of a product of sets.
> >
> > Seems like a subtype to me. Every place a relation value
> > could be used, a tuple value could be used instead.
> >
> > Note there is no circularity required.
> Ah, so a relation is not a set of tuples? What do you call an element of
> a relation, then?
If a relation is a relation and not a set then what do you call an element of a relation then ?
> And where do the attribute names and domains come into this definition?
> Do you use attribute ordering and typeless sets?
>> > part of 1. The heading is associated with the value, but it is
> > > That depends on which definition you consider "the" definition. I like
> > > Date's: A relation consists of a heading (a set of attribute name /
> > > domain pairs), and a body, which is a set of tuples conforming to the
> > > heading. A tuple is a set of attribute name / domain / value triplets.
> >
> > That definition includes the type as part of the value. The relation
> > heading is not part of the relation value, no more than "int" is
> > not a part of the value per se.
> Fair enough, but you have to specify the heading *somewhere*. A relation
> literal must necessarily somehow include attribute names (and types, but
> that is easier to do implicitly). Where/how do you do it?
In the =.
> > > As for subtypes, I'm undecided. I like the TTM subtype definition
> > > (subset; specialisation by constraint) because it is clean, precise
and
> > > sound---but it seems a bit useless in practise. :)
> >
> > Well, the "subset" part certainly applies.
> Well, given your definition, I agree that tuple is a subtype of a
> relation. It is just the question of whether I think your definition is
> good or not.
It is a description not a definition. Received on Tue Mar 07 2006 - 10:53:25 CET