Re: I think my book may be wrong about cardinality, but I'm not sure
Date: Tue, 24 Jul 2007 20:35:46 GMT
Message-ID: <Cotpi.7328$XL4.3907_at_trndny04>
"beginner16" <kaja_love160_at_yahoo.com> wrote in message
news:1185301676.680771.29780_at_b79g2000hse.googlegroups.com...
> hello
>
>
> 1)
> The following quote ( well I shortened it a bit ) is from a chapter
> briefly describing MARTIN E-R notation
>
I wonder if you shortened it correctly. Sometimes the diagram can make it look as if the cardinalities are written at the wrong end.
> "Say we have entities ORDER and PRODUCT. One ORDER must include at
> least one product, but it can also have more than one product. One
> PRODUCT can be related to zero or more ORDERS. Thus cardinality of
> PRODUCT is ( 1, N ) and cardinality of ORDER is ( 0, N )"
>
> But to my understanding, the cardinality of ORDER entity should be
> ( 1,N ) --> where 1 means min number of connections and N max number
> of connections an individual ORDER entity can have. And cardinality of
> PRODUCT entity should be ( 0,N ). But my book claims just the
> opposite!
>
>
I would have said it differently: the cardinality of the PRODUCT-ORDER
relationship with respect to ORDERS is (1,N). the cardinality of the
PRODUCT-ORDER relationship with respect to PRODUCTS is (0,N). I think you
and I are on the same page, but may need to reread Martin.
> 2)
> Relationship between two entities is called binary connection or
> second degree relationship. But connection can exists between more
> than just two entities. Level of connection is determined by the
> number of different entity types that exist in a connection.
>
> Now as far as relational DB goes, don't tables have only binary
> connections ( second degree relationship )?
>
> So if in real world three entities are related to each other, but
> relational DB only supports binary connections, do we at conceptual
> level ( using E-R diagrams ) represent relationships between these
> three entities as one connection ( ternary connection ), or do we
> decompose it into two or more binary relationships?
>
>
Not necessarily. Some E-R diagramming techniques use a diamond for
relationships that are more than binary. (arity higher than 2)
Received on Tue Jul 24 2007 - 22:35:46 CEST