I think my book may be wrong about cardinality, but I'm not sure

From: beginner16 <kaja_love160_at_yahoo.com>
Date: Tue, 24 Jul 2007 11:27:56 -0700
Message-ID: <1185301676.680771.29780_at_b79g2000hse.googlegroups.com>


The following quote ( well I shortened it a bit ) is from a chapter briefly describing MARTIN E-R notation

"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!

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?

thank you

cheers Received on Tue Jul 24 2007 - 20:27:56 CEST

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