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Re: weak entity

From: Ashutosh Galande <galande_at_uiuc.edu>
Date: Fri, 11 Oct 2002 12:02:10 -0700
Message-ID: <gcFp9.923$US2.10606@vixen.cso.uiuc.edu>


I totally agree with you. But what I don't understamd is, if adding external ID attribute to inherent attributes of an entity doesn't make it strong entity then we are fraught with a number of weak entities in real life. I have seen it to be a common practice to use ID attribute for entities. These attributes, as you noted are generated with additional processing outside the database. But still the ER diagram represents these entities as a strong entity. So why this anomaly?

"Bernard Peek" <bap_at_shrdlu.com> wrote in message news:z8Fj3lEg+vo9Ew5G_at_shrdlu.co.uk...
> In message <Gnxo9.34396$m7.318065_at_vixen.cso.uiuc.edu>, Ashutosh Galande
> <galande_at_uiuc.edu> writes
> >Hi,
> >
> >While modeling a real life system it seems that its almost always
possible
> >to convert a weak entity into a strong entity by adding an "ID" attribute
to
> >it. (where ID is an external unique number which might not be a natural
> >descriptor/attribute of the entity but still can be used to identify each
> >entity instance uniquely eg: flight ID, item ID, book No, etc) So is
there
> >any golden rule or guideline which states when an entity should be kept
weak
> >and when an external attribute should be/can be added to it?
>
> Adding an ID field does not make the entity any stronger. The strength
> of the entity depends on the data and the ID field is not part of the
> data. The entity may appear stronger but this is an illusion. Remember
> that an entity is a description of the logical structure of the data not
> the physical implementation.
>
> There is a golden rule. The ID field only helps if you can guarantee
> that it is unique and that a given ID value can only possibly identify
> one instance of the entity.
>
> The uniqueness requirement is easily met with most modern database
> systems.
>
> Making sure that a given ID value can only be associated with a single
> instance of the entity is hard. If you can solve this then your ID field
> is almost as effective as a natural key. It usually requires additional
> processing outside the database system.
>
>
>
>
> --
> Bernard Peek
> bap_at_shrdlu.com
> www.diversebooks.com: SF & Computing book reviews and more.....
>
> In search of cognoscenti
>
>
>
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Received on Fri Oct 11 2002 - 14:02:10 CDT

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