Re: weak entity
Date: Fri, 11 Oct 2002 12:02:10 -0700
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
> In message <Gnxo9.34396$m7.318065_at_vixen.cso.uiuc.edu>, Ashutosh Galande
> <galande_at_uiuc.edu> writes
> >While modeling a real life system it seems that its almost always
> >to convert a weak entity into a strong entity by adding an "ID" attribute
> >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
> >any golden rule or guideline which states when an entity should be kept
> >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
> 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
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Received on Fri Oct 11 2002 - 21:02:10 CEST