Re: Weak entity types

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Tue, 14 Aug 2007 19:06:00 -0300
Message-ID: <46c22709$0$4024$9a566e8b@news.aliant.net>

David Cressey wrote:

> "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
> news:46c1f5c1$0$4052$9a566e8b_at_news.aliant.net...
> beginner16 wrote:
>
>

>>uh, confused
>>
>>On Aug 12, 3:01 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>>
>>
>>>beginner16 wrote:
>>>
>>>
>>>>hello
>>>
>>>>a)
>>>>Weak entity type cannot be uniquely identified by its own attributes
>>>>alone and thus needs another entity to be uniquely identified.
>>>
>>>I suggest you stop and question the above statement.
>>
>>I'm just quoting from a text.

>
>
> Are you suggesting one should never question the content of texts?
>
>
>

>>>>So in relational model, every relation which has primary key made of
>>>>foreign key and perhaps some other attribute, is weak entity type?
>>>
>>>Basically, yes.
>>>
>>>
>>>
>>>>Ok, but I could instead of creating a foreign key create another
>>>>attribute  which could uniquely identify rows in a table. By
>>>>definition the relation would no longer be weak entity type --> there
>>>>has to be more to this --> perhaps it's more of a subjective thing?!
>>>
>>>The original definition you give above is absurd on its face. To be a
>>>relation, one must be able to uniquely identify each of its tuples by

>
> value.
>

>>I'm not sure I understand what you are trying to say. Above I stated
>>that if we have weak entity type, then we could create another unique
>>attribute ( and make it a part of that weak entity type ) just for the
>>purpose of uniquely identifying tuples --> in short we would make this
>>attribute a primary key --> then the definition of weak entity type
>>would no longer describe this relation.

>
>
> Exactly, and I gave an example of adding an arbitrary lookup table to
> the schema suddenly turning a so-called strong entity into a so-called
> weak entity.
>
>
>

>>But on the other hand entities describe real world objects and if an
>>existence of some type of object ( call it A ) in real world depends
>>on the existence of objects of another type, then  A  should be
>>considered weak, regardless of whether A type object can uniquely be
>>identified by its own attributes or  with the help of compound primary
>>key ( made from foreign key )

>
>
> If I have a relation that describes properties of relations, does it
> describe a 'real world object' ?
>
> An attribute doesn't make sense without the existence of some relation
> type or tuple type just as an order line item doesn't make sense without
> an order.
>
> Beg pardon Bob, but an attribute does make sense in the E-R model without
> any reference to a relation type or tuple type. From the rest of the OP use
> of terminology, I gather that he's quoting from a text that's developing
> the E-R model.

I thought I made it clear I was talking about a relation that describes properties of relations. I guess I was wrong. Received on Tue Aug 14 2007 - 17:06:00 CDT