Re: Database-valued attributes?
Date: Tue, 11 Nov 2003 11:21:59 -0000
Message-ID: <boqhlv$1226$1_at_gazette.almaden.ibm.com>
"Jonathan Leffler" <jleffler_at_earthlink.net> wrote in message
news:Ir%rb.7052$nz.2397_at_newsread2.news.pas.earthlink.net...
> Marshall Spight wrote:
>
> > There's been the occasional incidental discussion of relation-valued
> > attributes on the list lately. This has got me thinking: what about
> > database-valued attributes? That is to say, what about an attribute
> > that consists of a set of relation values?
>
> Hmmm...would the elements of the set of tables all need the same type?
> In the RMD (relational model of data), the values in a column are
> homogeneous in type, usually. For example, in a table with one or
> more RVAs, the RVAs for a given attribute have the same structure/type
> in each row.
>
> What is a database? That isn't intended to be wholly facetious.
No, indeed this is a very important question, and not one that to my knowledge that has an agreed answer. Well, I have an answer, but I don't believe that is it is widely agreed upon.
See this thread
http://groups.google.co.uk/groups?selm=3cccff99%241%40news.uia.ac.be
In particular, Jan's comments:
"a database instance is not a set of relations but more like a tuple of
relations"
"I have seen lots of formal definitions of a relational database and not one
of them defined a schema as a set (as in set theory) of tables (or
relations, to be more exact)."
Oh, my answer is that
a database is a set of tuples.
nice and simple.
:-)
P.S. That's tuples not necessarily of the same type of course.
Regards
Paul Vernon
Business Intelligence, IBM Global Services