Re: Relation Schemata vs. Relation Variables

David Cressey wrote:
> <brian_at_selzer-software.com> wrote in message
> news:1156344427.551602.122270_at_h48g2000cwc.googlegroups.com...
>
>
> > A candidate key value can only identify a tuple within a single
> > database state.
>
> Here's where you're going wrong, Brian. A candidate key (from the logical
> model) doesn't identify
> a tuple. Not even within a single database state.
>

Then why did Codd use exactly that term when he defined the Relational Model? I quote,

>>>>

Normally, one domain (or combination of domains) of a given relation has values which uniquely identify each element (n-tuple) of that relation. Such a domain (or combination) is called a primary key.*

*E. F. Codd, "A Relational Model of Data for Large Shared Data Banks," Communications of the ACM (June 1970)

<<<<

>
> One might say that it specifies a tuple. That isn't the same thing as
> identifying the tuple.
Received on Thu Aug 24 2006 - 18:31:43 CEST