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Re: c.d.theory glossary - NULL

From: Alfredo Novoa <alfredo_at_ncs.es>
Date: Tue, 27 Apr 2004 16:45:22 GMT
Message-ID: <408e8bee.24045345@news.wanadoo.es>


On Tue, 27 Apr 2004 12:18:40 -0400, "Laconic2" <laconic2_at_comcast.net> wrote:

>And here's another difference between a logical model and a physical model.
>In a logical model,
>if you are using relations as your modeling tool, then NULLS should not be
>allowed. The reason?
>NULLS can't exist in a relation.

Agreed. Without nulls we are applying First Order Logic, with nulls we are applying ??? :)

>In the physical model, if you a modeling in terms of tables, rows and
>columns, there's no real reason to forbid NULLS in every column.

But those "nulls" are very different to the SQL nulls. SQL nulls are logical, not physical.

> Some
>columns should have NULLS forbidden in order to maintain entity integrity,
>but optional data is a possibility.

You can implement the integrity constraints in many ways, with or without marks in the physical tables, if you have physical tables at all.

Regards
  Alfredo Received on Tue Apr 27 2004 - 11:45:22 CDT

Original text of this message

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