Re: Plural or singular table names
Date: Wed, 3 Sep 2003 21:55:01 -0400
Message-ID: <m6y5b.465$EE4.45821030_at_mantis.golden.net>
"Marshall Spight" <mspight_at_dnai.com> wrote in message news:0sv5b.163742$2x.45738_at_rwcrnsc52.ops.asp.att.net...
> "Bob Badour" <bbadour_at_golden.net> wrote in message
news:M9s5b.436$iP3.44134429_at_mantis.golden.net...
> >
> > > A relation value has no information about constraints.
> >
> > I disagree. Every relation has at least one candidate key, and the
candidate
> > keys can affect the result values of operations.
>
> I notice you are not specifying relcon vs. relvar, which I conclude
> (from the fact that you're generally precise on these matters, and
> from context) that you mean both.
>
> But I'm unable to figure out how what you're saying might apply to
> a relcon. Can you give me an example of an operation on one or
> two relational values and an associated operator that would be
> affected by the presence of a candidate key?
The candidate keys affect the type of the result of just about every
operation. Consider two ternary relations that each have attributes A, B,
and C where the only candidate key of one relation is { A } and the only
candidate key of the other relation is { C }. Consider the result of the
project operation over { B, C }
The candidate key of the project will be { B, C } for the first relation and
{ C } for the second relation. The results have different types. Now, it is
possible that both relations contain the same set of tuples where { C } is
incidentally unique for the first relation and where { A } is incidentally
unique for the second. Even though the set of tuples in both operands and
the set of tuples in both results are the same, the types differ.
> I guess I'm also assuming (much less solidly) that when you say
> candidate key, here, you are saying something *beyond* the
> fact that the rows must be unique. We would all agree (I hope!)
> that the members of a relation value are unique; I generally
> consider that part of the definition of relation, rather than
> that the definition includes the requirement of at least one
> candidate key, and that uniqueness is a consequence of
> that key.
The two definitions are equivalent. Every relation having at least one irreducible key implies unique tuples and vice versa. Received on Thu Sep 04 2003 - 03:55:01 CEST