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Re: Normalisation

From: Jon Heggland <>
Date: Tue, 5 Jul 2005 12:41:16 +0200
Message-ID: <>

In article <oRfye.137238$>, says...
> >>Roughly, I would say that it means that all your data manipulation can
> >>be expressed as operations over a signature that consists only of (1)
> >>domains described by an abstract data type and (2) relations that
> >>contain in their fields only values from these domains.
> >
> > I can't make heads or tails of this. Operations over a signature?
> Yes, that means that they can be described in a programming language in
> which your access to the database is restriced to the functions in the
> signature. These operations would be the (1) the operations that are
> associated with the domains and (2) operations that retrieve the current
> instance of a relvar.

How does a set domain (e.g. the domain of sets of integers) violate this? For definiteness, let's associate the normal set operations with it---union, intersection, subset, cardinality and so on.

(By the way, is you phrase "domains described by an abstract data type" significant? If so, can you explain what it means? Are you talking about type generators like set, array, list and so on?)

> >>An unnest operation, for example, cannot be described in that way.
> >
> > What if I define an unnest operator that "unnests" strings in a
> > corresponding manner?
> It does not satisfy the definition.

Does that mean that I should not be allowed to define such an operator?

Or that strings are not atomic?

Or that string are atomic and should not be normalised unless and until I define an unnest_string operator, in which case all my relvars with strings suddenly are no longer in 1NF?

Does it matter whether my unnest_string is system defined (by the DBMS) or user-defined?

> > Anyway, let me rephrase my initial question.
> > What is an atomic datatype/domain?
> The problem with that question is that it makes an incorrect assumption.
> Atomicity is not a property of a domain per se,

That is news to me. All database textbooks I have used use the phrase "atomic value" to define 1NF. Which, while obviously different from "atomic domain", can hardly be interpreted very differently. I also note that most of them concur with Date that 1NF is implied by the definition of a relation.

Where do you get your 1NF definition from?

> but rather a property of
> how it is treated by certain operations. The more correct question is:
> "When does the DBMS treat a certain domain as atomic?". You can derive
> the answer to that question from what I said above.

How does that help with normalisation? And how can you say a priori that a relvar with a set-valued attribute is not in 1NF, if that depends on the operators of a particular DBMS?

Received on Tue Jul 05 2005 - 05:41:16 CDT

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