Re: Weak Entity

From: Peter Kwok <peterkwok628_at_yahoo.com>
Date: Mon, 14 Oct 2002 10:16:35 +0800
Message-ID: <aod8v1$5qi$1_at_news.ctimail.com>


> >In ER diagram, should weak entity be related to strong entity in 1-N
> >relationship?
> >i.e.
> >Correct:
> >Strong Entity----------(1)-Relate-(N)----------Weak Entity
> >
> >Wrong:
> >Strong Entity----------(M)-Relate-(N)----------Weak Entity
>
> No.
>
> For example:
>
> Element --(M)-Is_contained_in-(N)-- Non-empty_Set
>
> Clearly non-empty sets cannot exist without their elements and they have
no
> attributes to form a key.
>
> However, some definitions of the ER model do have this restriction, so it
> also depends a bit on what book you are using and how smart your
instructor
> is. :-)
>
> -- Jan Hidders
>
>

However, how can weak NON_EMPTY_SET be uniquely identified by relating to its partner-ELEMENT under such many-many relationship? //Peter Received on Mon Oct 14 2002 - 04:16:35 CEST

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