RL notation

From: Tegiri Nenashi <TegiriNenashi_at_gmail.com>
Date: Thu, 7 Feb 2008 12:21:17 -0800 (PST)
Message-ID: <24f87f1a-a741-4438-9e6f-dc0466ff8194_at_s13g2000prd.googlegroups.com>



Marshall wrote:
> Brass tacks:
>
> X(x) -- the domain
> R(a, b) -- the relation
>
> X & (x=a) & (x=b) => R
>
> X is a relation with one attribute, x.
> R is a relation with attributes a and b, taken from
> the members of X
>
> The phrase "(x=a)" denotes the infinite relation of attributes
> a and x where a = x. In set builder notation:
>
> {(x, a) | x in X and x = a}
>
> "&" is natural join. So
>
> X & (x=a)
>
> joins X with (x=a), creating a new relation with attributes x and a.
>
> X & (x=a) & (x=b)
>
> does the same thing, so now we have
>
> {(x, a, b) | x in X and x = a and x = b}
>
> "=>" is the generalized subset operator. It takes two relation
> operands, and evaluates to true if the left operand has a
> superset of the attributes, and a subset of the elements
> of the right operand.

I suspect only 3 people in the world are comfortable with this notation:-) It is imperfect for at least two reasons: 1. Equality relation, is it some kind of constant in the RA? Also the "=" symbol in the (a=b) conflicts with the equality among relations. Two levels of equality is something people never dealt before? 2. Explicit column variables. Having two sorts of symbols -- realtion and attribute variables -- makes RL expressions less pretty. Received on Thu Feb 07 2008 - 21:21:17 CET

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