Re: About Entity Relation Diagram

From: Gene Wirchenko <genew_at_mail.ocis.net>
Date: Thu, 16 Dec 2004 08:31:53 -0800
Message-ID: <nmd3s0tioi7mscglip37208g0u45g5v3tt_at_4ax.com>


"Silver" <argytzak_at_med.auth.gr> wrote:

>I thought that normalizing was all about making flat tables into tables with

                                                 ^^^^
     Get rid of this word.  Relations are not flat.  A relation is
n-dimensional where n is the number of attributes.

>as less columns as possible.

    ^^^^
     "few".

     No, it is about removing certain kinds of redundancy.

     If you need a wide table, you need a wide table.

>I do, however, have another question. What will the primary key of the
>entity INTERESTS and APPEARANCE be? I read somewhere that when relating 2
>entities with a relation, the relation doesn't have to include the primary
>keys of the entities it relates. For example, between member and tape, the
>relation RENTAL doesn't have to include MemverID and TapeID, only DateRent
>and DateDue.

     This is not correct. You need to have the keys. What you do not do is show them when at the logical level. They are implied. (Think of how you would use that relation. You need to have the keys.)

>Will this be the same case with the entities Member and Interests ? (the
>relation will be HAS). But then, what attributes will the relation HAS have?

     It appears so.

     It might not have any non-key attributes. Such a relation can be perfectly legitimate.

>PS. I 'm also reading some theory stuff. I just could use an extra advice

     Carry on.

[snipped previous]

Sincerely,

Gene Wirchenko

Computerese Irregular Verb Conjugation:

     I have preferences.
     You have biases.
     He/She has prejudices.
Received on Thu Dec 16 2004 - 17:31:53 CET

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