Re: thinking about UPDATE

On Thu, 22 Jul 2004 12:05:30 -0700, Mikito Harakiri wrote:
>
> If we have
>
> {}->{A}
>
> FD, then, column A should have identical values, right?

Yes.

> On the other hand, {}->{A} trivially holds as Armstrong reflexivity rule.

An FD X->Y is trivial if Y is a subset of X. So {A}->{} is trivial but {}->{A} is not.

• Jan Hidders