Re: more closed-world chatter

On May 11, 3:18 am, Jon Heggland <jon.heggl..._at_idi.ntnu.no> wrote:
> Vadim Tropashko wrote:
> > I suggest [anti]join as a generalized minus. The generalized Date
> > identity is:
>
> > (A /\ B) \/ (A /\ 00) = A \ (A \ B)
>
> > where /\ is join, \/ is generalized union, and \ is antijoin.
>
> So the left side is a semijoin, right?

I see. The term semijoin applies to "A \ (A \ B)", while the term for "A \ B" is antijoin.

> What do you mean by the phrase
> "The generalized Date identity"?

Well, the sets identity

A /\ B = A \ (A \ B)

is a special case of

(A /\ B) \/ (A /\ 00) = A \ (A \ B)

hence the term. I have no doubt it has been known since ancient times, but I'm not aware if anybody put an emphasis to it like Date did. Received on Fri May 11 2007 - 18:09:26 CEST