Re: thinking about UPDATE

On Tue, 27 Jul 2004 09:13:54 +0300, x wrote:
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
> news:pan.2004.07.26.17.41.23.393221_at_REMOVETHIS.pandora.be...

>> On Mon, 26 Jul 2004 16:07:11 +0300, x wrote:
>> > What made you think this:
>> > "However, the only FDs that hold in the original relation are those
>> > that are implied by its CKs"

>> That's part of the theorem that is being proven. Try writing it out in
>> full (that's not as easy as you perhaps would expect) and you will see
>> what I mean.

Of course, but I'm not starting with *any* relation. To show the type of completeness that we are dealing with here it is sufficient to show that for every set A of attributes that is not derived by the algorithm there is a relation that satisifies the initial set of candidate keys and in whose projection A is not a candidate key. Since the only restriction is that it satisfies the original set of candidate keys I'm free to assume that the only FDs that hold for it are those that are implied by these candidate keys.

You are right that I oversimplified the proof, but my goal was to get you thinking about it, and it seems to have worked. :-)

• Jan Hidders