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Re: How long is long enough for primary key ?

From: David Portas <REMOVE_BEFORE_REPLYING_dportas_at_acm.org>
Date: 7 Oct 2006 10:51:30 -0700
Message-ID: <1160243490.409610.73690@i42g2000cwa.googlegroups.com>


DA Morgan wrote:
>
> No. But it is rather hard to photocopy Codd's books and post them here.
>
> Perhaps you'd prefer one of these instead:
> http://www.cdf.toronto.edu/~boconnel/434/normalization.txt
> http://www.devx.com/ibm/Article/20859
> http://www.cs.jcu.edu.au/Subjects/cp1500/1998/Lecture_Notes/normalisation/2nf.html
> http://www.utexas.edu/its/windows/database/datamodeling/rm/rm7.html
>
> I've got a couple of hundred more if these similarly produce fits of
> laughter.

But none of those sources supports your assertion about composite primary keys. They all contradict you!

"A relation is in 2NF if it is in 1NF and every non-key attribute is fully dependent on each candidate key of the relation" http://www.cs.jcu.edu.au/Subjects/cp1500/1998/Lecture_Notes/normalisation/2nf.html

In other words 2NF (like all normal forms) is concerned with Candidate Keys not Primary Keys. 2NF does not disallow composite keys. I can't lookup Codd's normalization paper right now but here's an equivalent definition:

"A relation scheme R is in second normal form (2NF) with respect to a set of FDs F if it is in 1NF and every nonprime attribute is fully dependent on every key of R."

Maier, The Theory of Relational Databases, p99.

-- 
David Portas
Received on Sat Oct 07 2006 - 12:51:30 CDT

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