Re: DBDesign Q

Very briefly. An ERD models business entities and relationships, and there can be > 1 relationship between the same enities. When you map the ERD to a realtional schema these multiple relationships combine into non-redundant tables.

"Bob Badour" <bbadour_at_golden.net> wrote in message news:rtGdnVlFq_Dx1l-iRVn-ig_at_golden.net...
> "Alan" <alan_at_erols.com> wrote in message
> news:bpthmj$1t348d$1_at_ID-114862.news.uni-berlin.de...
> > It is correct on an ERD, but incorrect in a relational schema.
>
> That's a rather astounding statement. How do you propose to support your
> assertion?
>
>
> > "Andy" <net__space_at_hotmail.com> wrote in message
> > news:edb90340.0311232036.e0eeae5_at_posting.google.com...
> > > Can two entities have more than one relationship between them?
> > >
> > > A(A_ID PK,..., bID FK)
> > >
> > > B(B_ID PK,..., aID FK)
> > >
> > > A.bID refers to B.B_ID
> > > B.aID refers to A_ID
> > >
> > > 2. Does it make sense?
> > >
> > > Product (ProductID PK, Name,., SpecialAttrID FK)
> > >
> > > Attribute (AttrID PK, Name,..,ProductID FK)
> > >
> > > Product can have many attributes. One attribute belongs to only one
> > product.
> > > There is only one main("special") attribute for each product
> > >
> > > Thank you in advance,
> > > Andy
> >
> >
>
>
Received on Mon Nov 24 2003 - 22:36:14 CET