Re: Identifying Relation vs Non-Identifying?

From: Dan <dan_at_nospam.com>
Date: Tue, 05 Jul 2005 11:29:46 -0500
Message-ID: <%zyye.10$JM6.672_at_news.uswest.net>

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? I came across these terms
>>> when I recently started looking into using ERM tool.
>>
>>
>>
>> Don't you mean an "identifying relation*ship*"? Be careful not to
>> confuse the notions of relationship as used in ERDs and relation as
>> used in the Relational Model.
>>

> 
> The 2 tools call it a "relation".  Since it is for drawing a line 
> between tables (relations), I guess they should call it "relationship".
> 

>> Loosely speaking an identifying relationship is a relationship that is
>> necessary to identify a certain entity that plays a role in it.
>> Consider for example the relationship "owns" between "Person" and
>> "Dog" where "Dog" only has attribute "name". In that case a dog is
>> probably not uniquely identified by just its name, but it will be by a
>> combination of name and owner. In some sense you could say that the
>> relationship "owns" is part of the key of "Dog". And that makes it an
>> identifying relationship. Also google for "weak entity".

> 
> 
> Makes sense to me.

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?

Dan Received on Tue Jul 05 2005 - 18:29:46 CEST