Re: Identifying Relation vs Non-Identifying?
From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Tue, 05 Jul 2005 19:58:33 GMT
Message-ID: <JDBye.138036$KB5.7268463_at_phobos.telenet-ops.be>
>
> Can one just say that a weak entity is a tuple whose primary key relies
> on the primary key of another entity? And that this type of
> relationship and only this type is then defined as a weak relation?
Date: Tue, 05 Jul 2005 19:58:33 GMT
Message-ID: <JDBye.138036$KB5.7268463_at_phobos.telenet-ops.be>
Dan wrote:
> On 7/5/2005 10:57 AM, Dan wrote:
>
>> On 7/1/2005 6:27 PM, Jan Hidders wrote: >> >>> Dan wrote: >>>> >>>> Can someone explain to be me the difference to between an >>>> identifying relation and a non-identifying relation? [...] >>> >>> Loosely speaking an identifying relationship is a relationship that >>> is necessary to identify a certain entity that plays a role in it. >>> [...]
>
> Can one just say that a weak entity is a tuple whose primary key relies
> on the primary key of another entity? And that this type of
> relationship and only this type is then defined as a weak relation?
Yes. That is how you usually map such a construct in an ER diagram to relations. The PK of the weak entity will consist of the local identifying attrubutes plus the PK of the entity it depends upon. Things can get a bit more complicated if the identifying relationship is a many-to-many relationship, but that situation is very rare and some even disallow it.
- Jan Hidders