Re: thinking about UPDATE

On Fri, 23 Jul 2004 01:43:23 +0000, D Guntermann wrote:
>
> Well, I think this is what I am getting at, ultimately. Something really
> bothers me about the use of the empty set in FD's. I can't quite place my
> finger on it, but it seems inconsistent by some rules and definitions.

That's because you seem to be treating {} as if it is an attribute, which it really isn't. Could it be that you are confusing the informal and formal notation for FDs? Note that informally we often write things like A->B as a short-hand for {A}->{B}, and at other times we write X->Y and really mean X->Y because X and Y are sets. So perhaps when you read things like {}->Y you somehow go the idea that this really meant {{}}->{Y}?

So let's now write all the braces to avoid confusion. Take a relation R(a,b,c) and consider some FDs that correspond to different CKs:

CK {a,b,c} corresponds to FD {a,b,c}->{}
CK {a,b}   corresponds to FD {a,b}->{c}
CK {a}     corresponds to FD {a}->{b,c}
CK {}      corresponds to FD {}->{a,b,c}

Note that the last line does not say FD {{}}->{a,b,c} because that wouldn't make any sense; FDs hold between sets of attributes and the empty set is not an attribute.

Does this compute? :-)

• Jan Hidders