repeating groups (was: something with nVL and NULL)

From: mAsterdam <mAsterdam_at_vrijdag.org>
Date: Sun, 19 Feb 2006 02:11:22 +0100
Message-ID: <43f7c540$0$11064$e4fe514c@news.xs4all.nl>

Marshall Spight wrote:
> David Cressey wrote:
>

>>The problem, in this case, is that current terminology with regard to 1NF is
>>divided, among data modellers, on precisely the point that Dawn wishes to
>>discuss:  the elimination of repeating groups as a first step in normalizing
>>a schema or relations.

>
> Yeah, I hear that. I guess at least part of what I'm responding to
> is that she completely threw me off when she used the term that
> way.
>

>>According to the way many of us (I include myself) use the terminology,
>>putting a relation into 1NF implies decomposing, when necessary, in order to
>>express repeating groups as sets of tuples, in separate relations.

>
> Sure. But there are several ways out of the repeating groups problem
>
> 1) decomposing relations, aka "classical" 1NF
> 2) higher-than-1 cardinality attributes: lists or sets

Aren't you jumping over the "order may have meaning" problem here by taking these two together?

> 3) Fully nested relations.

lists-of-lists? sets-of-sets? lists-of-sets? sets-of-lists?

> Actually, 2) was something I hadn't really thought of before:
> add *one* level of nesting. This idea is interesting but less
> appealing than 3, because it is less regular. I tend to prefer
> orthogonal designs; they have fewer arbitrary limitations.
>
> The thing that I can't help but feel, though, is that the whole
> thing is a false dichotomy. If you can really always do the
> composition/decomposition, then it would appear that the
> choice between nested structure or not could be expressed
> simply as a view. How come no one ever talks about that
> idea?

When I try to make an example, sooner or later I find myself somehow inventing new user relevant meaning (which IMO is wrong) - but this just may be because I can't come up with the right examples.

[snip agreement on Date's definition] Received on Sat Feb 18 2006 - 19:11:22 CST