Re: Does entity integrity imply entity identity?
Date: Sun, 2 Aug 2009 17:51:28 -0400
"David Portas" <REMOVE_BEFORE_REPLYING_dportas_at_acm.org> wrote in message
> "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
>> Mr. Scott wrote: >> >>> "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message >>> news:4a7479f3$0$23783$9a566e8b_at_news.aliant.net... >>> >>>>Mr. Scott wrote: >>>> >>>>>Since the entity integrity rule ensures that a relational table cannot >>>>>have any duplicate rows, does that imply that each row in a table maps >>>>>to a distinct entity? >>>> >>>>I am unfamiliar with an entity integrity rule. It sounds like some shit >>>>somone just made up to market ER diagrams. >>> >>> I thought Codd referred to the the entity integrity and referential >>> integrity rules as the insert-update-delete rules of the relational >>> model. >> >> Can you cite a reference for that? >> >> >
> The "RM/T" paper, Extending the Database Relational Model to Capture More
> Meaning, ACM TODS, Vol. 4, No. 4, December 1979:
> "Rule 1 (entity integrity): No primary key value of a base relation is
> allowed to be null or to have a null component."
> It therefore originates from the point at which Codd decided to "extend"
> the model with E-relations, nulls and other exotic things. The rule begs
> more questions than it answers (what about derived relations and relations
> with multiple keys?). Myself and likeminded people are happy with the
> concept of a pre-RM/T relation that needs no such rules.
At the time that the "RM/T" paper was published (December, 1979), nulls had already been around for several years. They predate even Chen's E/R model. In January of 1975, Codd wrote, "A basic integrity principle associated with candidate keys is that, for every base relation, at least one of the candidate keys is prohibited from taking on null values." "Understanding Relations (Installment #6)," ACM SIGMOD Record, Volume 7, Number 1, Page 1. In the "RM/T" paper, Codd refers to the entity integrity rule and the referential integrity rule as if they had already been part of the then-existing relational model rather than part of the extended RM/T. Later on in 1975, in "Understanding Relations (Installment #7)," ACM SIGMOD Record, Volume 7, No. 3-4, Pages 24-28, Codd outlined the three-valued logic and the effect of nulls on the operators of the relational algebra that is restated in section 2 of the "RM/T" paper. Received on Sun Aug 02 2009 - 23:51:28 CEST