Re: How can I proove associtivity of natural in relational algebra?

From: Mikito Harakiri <mikharakiri_nospaum_at_yahoo.com>
Date: 22 Nov 2005 09:03:15 -0800
Message-ID: <1132678995.297690.79410@g47g2000cwa.googlegroups.com>

Jon Heggland wrote:
> In article <1132608953.263060.39140_at_g44g2000cwa.googlegroups.com>,
> mikharakiri_nospaum_at_yahoo.com says...
> > "Extend", is the opposite of "project". E.g. the relation with a single
> > attribute
> >
> > A = {(a=1), (a=2)} is exended to the attribute set {a,b} with the
> > domain of attribute b being {7,8,9} as
> >
> > A' = {(a=1, b=7),(a=1, b=8),(a=1, b=9),(a=2, b=7),(a=2, b=8),(a=2,
> > b=9))}
>
> This looks like a cartesian product to me. Why do you call it "extend"?

Oh, it is cartesian product. I was thinking along extend-summarise, but CP is much simpler idea. Received on Tue Nov 22 2005 - 11:03:15 CST