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

"beginner16" <kaja_love160@xxxxxxxxx> wrote in message
news:1185316444.726002.242770@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

No. An N-degree table implements an N-ary relationship between
attributes -
attributes which may well identify other entities. Do not assume
relationship = foreign key. A foreign key is just one type of constraint
(not necessarily the only one) for enforcing referential integrity in RM.


I'm not sure I understand what you are saying here!
I know that for ternary relationship we can create additional table
with three foreign keys ( which in a way is still made of two binary
connections ), but are you saying we can create ternary relationship
without foreign key constrains?


The point is that relationships of whatever kind are represented by the
values within the tables themselves. The foreign key is one of several
constraints that may or may not exist to keep order - so to speak - but a
foreign key is just the name given to one fairly restrictive type of
constraint. For example a foreign key isn't good enough to enforce an
N-cardinality relationship, where N is some fixed value greater than 1 - so
you would need some other constraint(s) for that purpose. That doesn't mean
it isn't a relationship.

As a matter of detail the constraint that SQL calls a "foreign key" differs
in some subtle ways from what the relational model calls a foreign key. The
relational definition is that a foreign key references only a candidate key,
whereas SQL permits "foreign keys" to reference super-keys as well.

--
David Portas


.

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