Re: thinking about UPDATE
From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Thu, 22 Jul 2004 19:14:32 GMT
Message-ID: <pan.2004.07.22.19.15.21.916247_at_REMOVETHIS.pandora.be>
Date: Thu, 22 Jul 2004 19:14:32 GMT
Message-ID: <pan.2004.07.22.19.15.21.916247_at_REMOVETHIS.pandora.be>
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