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Re: Why spurious tuples with fifth normal form?

From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Wed, 15 Jun 2005 23:23:13 GMT
Message-ID: <BL2se.121808$tN5.6787205@phobos.telenet-ops.be>


stowellt_at_gmail.com wrote:
> Here is some more info:
>
> The original table before I decomposed it was like this:
>
> parameter_id // FK to a list of chemical parameters (silver, gold,
> etc.)
> classification_id // FK to a list of classifications (drinking water,
> irrigation water, livestock
> url_id // FK to a list of url links to web sites

Does parameter_id identify a single parameter or a list of parameters (which is what you seem to say in your comment)?

At first sight this looks very much like it's already in 5NF, but of course this all depends on which functional dependencies (FDs), multi-valued dependencies (MVDs) and join-dependencies (JDs) you think hold. Can you tell us a bit more about that?

> If I then re-join all three tables, I get spurious results. I am
> stumped as to the problem. did I decompse wrong? There is a complete
> many-to-many-to-many relationship between each of the three FK's.

You mean that with every parameter_id there can be multiple classification-id's associated, et cetera? That only tells you that there are no FDs betweeen the individual columns. There could still be FDs between combinations of columns, and it says absolutely nothing about the possiblity of MVDs and JDs.

If you want to check for MVDs (multi-valued dep.'s) then the relevant question is if these relationship are independent, for example if the fact that a parameter_id is associated with a certain classification_id is completely independent of any involved url_id.

Received on Wed Jun 15 2005 - 18:23:13 CDT

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