Re: more on delete from join

paul c wrote:
...
> Yes, it's true that NOT (Pabc AND Rabd) by itself doesn't imply NOT Pabc
> AND NOT Rabd. However, "Pab = Rab AND NOT (Pab AND Rab)" does imply the
> conjunction NOT Pab AND NOT Rab. In other words, we can retract equal
> projections of subsets of Pabc and Rabd if we can interpret a delete
> statement in terms of Pab and Rab.
> ...

That last sentence might be wrong, can't find the right back of an envelope at the moment, running short of blanks since I have my mail delivered to an address many miles away. I might have been thinking of products, not projections, need to think again. But the second sentence still seems essential to my attitude. The posts about the general problem being underconstrained are all very interesting even though I haven't learned how to use Proover9 but I've been assuming various additional constraints like that one all along. Received on Tue Sep 01 2009 - 21:57:21 CEST