Re: teaching relational basics to people, questions

From: Sampo Syreeni <decoy_at_iki.fi>
Date: Thu, 19 Nov 2009 01:32:41 -0800 (PST)
Message-ID: <fb7d0941-b96e-4520-a898-d687998c77ba_at_o10g2000yqa.googlegroups.com>


On Nov 19, 5:56 am, com..._at_hotmail.com wrote:

> But the relation in the example has more than one non-key attribute.
> So that collection of relations and constraints is not in 6NF.

The definition of 6NF is that the schema must not satisfy any nontrivial  join dependencies at all. This implies at most one non-prime attribute per relation. As I see it, the condition that there be at most one non-prime attribute is a constraint on the relation in the usual sense. It does not follow from key and domain constraints, however. Hence, the relation may not be in 6NF, but I don't think it is in DK/NF either. If so, it isn't a counter-example to DK/NF implying 6NF like Date seems to think.

> It would probably help if you didn't use vague and/or suggestive
> (thus, meaningless) phrases like "logically subsumed".

At least that one has a perfectly standard meaning in mathematics. It's strictly the same as "is implied by".

--
Sampo
Received on Thu Nov 19 2009 - 10:32:41 CET

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