Re: cdt glossary 0.1.0 [Relation]
Date: 25 Feb 2006 05:20:12 -0800
Message-ID: <1140873612.463433.166760_at_z34g2000cwc.googlegroups.com>
Mark Johnson wrote:
> [Relation]
> 1. A relation is a subset of the set of ordered
> tuples (A1, A2, ... Am) formed by the Cartesian
> cross-product of sets S1 x ... x Sm where each
> An is an element of Sn.
>
> I might say, instead, because I think they logically go closely
> together, and again just as a suggestion, nothing more:
Mark -- I, too, am unable to understand your intended meaning from what
you write, in general. Can you spell out what you mean by the above
sentence?
> Relation - mathematically by a language of sets, a relation is an
> (ordered) array/tuple of elements drawn one each from a set of
> domains, in order, with duplicate domains allowed. This set of
> n-tuples, said to have degree, n, or to be 'n-ary', where the number
> of n-tuples is said to be its, cardinality. The relation is uniquely
> named with regard to other relations. It can both be imagined and
> represented as a 2-D grid, or table.
This definition does not seem better than the one that I think you are
suggesting be replaced. There are multiple definitions of Relation in
the glossary. In order for me to understand your point, could you say
what you think is missing from, or less than clear in, the current
definitions?
> Tuple - also termed an, element, of a relation, it is an unordered set
> of n-attributes,
this might seem like nitpicking, but one part of your style that is
difficult to read is your use of commas between an article and a noun.
Removing some commas might help. For example, the above fragment could
be punctuated as "Also termed an element of a relation, it is an
unordered set of n-attributes,"
Once I was able to read it that way, I could better identify whether I
agreed or disagreed with it. In this case, I disagree. 1. A tuple is
not a set. 2. I don't know what an n-attribute is. 3. Tuples are not
unordered. The combination of poor grammar and punctuation along with
imprecise terminology makes it very difficult for me to read, and I am
a native speaker of English. There a many readers where English is a
second language and I'm sure this makes it very difficult for them to
understand you too.
> domain, as irreducible 'atomic' scalars or strings (though multiples,
> if a multi-valued domain, were once considered),
We do not need to introduce into a definition of a tuple any discussion
about what domains might be valid in database products.
> unique name in order to a) express any relationship among any set of
> attributes and b) to allow the tuple to have no particular order. As
> an unordered tuple does not properly correspond with a set relation,
> the relation might be termed, instead, a relationship, which might
> more properly correspond with a table. The tuple can be imagined as a
> vector/array. The tuple can be imagined and represented as a rows in
> the table, its attributes as columns.