Re: Examples for one-to-one associations?
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Fri, 18 Jun 2010 17:49:57 -0300
Message-ID: <4c1bdb7c$0$11837$9a566e8b_at_news.aliant.net>
>
> I don't think that is correct. The cardinality is defined by the
> domain definitions of the relation, not by the current extent of
> the relation. Further, since we are not dealing with multivalued
> theory the cardinality is never of the form "C1:C2 or C1:C3 ..."
> for a particular domain and relation (where C# are cardinals);
> rather it is fixed at some C1:C2 regardless of the current
> extension.
>
> As far as I know, in mathematics the properties "injective" (ie
> one-to-one) and "partial" are not mutually exclusive. Please Google
> "partial injective function" to find both definitions and examples
> of such functions. For example, the square root function over the
> natural numbers is considered a partial injective function.
Date: Fri, 18 Jun 2010 17:49:57 -0300
Message-ID: <4c1bdb7c$0$11837$9a566e8b_at_news.aliant.net>
Keith H Duggar wrote:
> "partial injective function"
>
> On Jun 8, 9:29 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
>>Keith H Duggar wrote: >> >>>On Jun 7, 2:25 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote: >> >>>>Gene Wirchenko wrote: >> >>>>>On Sat, 5 Jun 2010 20:49:59 -0700 (PDT), hoodwill >>>>><chase.saund..._at_gmail.com> wrote: >> >>>>>>Examples of one to ones: >> >>>>>>1) Spouse >> >>>>> You need a time element. Even given monogamy, many people have >>>>>been married more than once. >> >>>>Even more basic than that: Some folks are single. >> >>>Why is that an issue? If a person is single then they would not appear >>>in the Spouse relation. one-to-one relations can be partial can they >>>not? >> >>The relative cardinality is 1:zero-or-one not 1:1.
>
> I don't think that is correct. The cardinality is defined by the
> domain definitions of the relation, not by the current extent of
> the relation. Further, since we are not dealing with multivalued
> theory the cardinality is never of the form "C1:C2 or C1:C3 ..."
> for a particular domain and relation (where C# are cardinals);
> rather it is fixed at some C1:C2 regardless of the current
> extension.
>
> As far as I know, in mathematics the properties "injective" (ie
> one-to-one) and "partial" are not mutually exclusive. Please Google
> "partial injective function" to find both definitions and examples
> of such functions. For example, the square root function over the
> natural numbers is considered a partial injective function.
The original question was not about functions (or relations for that matter) but about "associations" and "entities".
Most (if not all) ER notations have distinct syntax for 1:1. 1:zero_or_1, 1:0_or_more, 1:1_or_more, 0_or_more:0_or_more etc.
0_or_more is your standard "many". Received on Fri Jun 18 2010 - 22:49:57 CEST