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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> 





hello




1)


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!




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?




thank you



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












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