Re: Weak entity types

"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. Received on Tue Aug 14 2007 - 23:48:07 CEST