Re: A different definition of MINUS, Part 3
Date: Wed, 17 Dec 2008 12:29:12 -0800
paul c wrote:
> Neither has the Y' result dropped any tuples and we have a situation
> where R = X <AND> Y, but R' is NOT equal to X' <AND> Y', in other words
> DELETE R WHERE X = 1 AND Y = 1 does nothing! ...
Oops, just noticed, that was pretty clumsy, what I wrote denies everything I've been saying all along and I didn't mean that. I should have said it subjunctively: "... if we had a situation where R' were not equal to X' <AND> Y', we would have a serious contradiction in the algebra, so R' IS equal to X' <AND> Y' and no tuples are dropped from the result...". I stand by the rest, ie., the DELETE statement does nothing.
(I don't know why Date referred to the statement as an "operation". Personally, I think calling it an operation takes a turn that might risk confusing a language implementation with the algebra the language interprets or, equally, is based on. But as long as I live, I imagine there will always be nuances he suggests that will puzzle me.)
Also, I want to comment further on what 'BS' said in a reply about an algebra and it not having any notion of 'before' or 'after'. I agree that the A-algebra doesn't have such notions, which in fact is one of its advantages, eg., there are fewer concepts in an algebra to confuse us (and I do mean the plural 'us'). As Darwen likes to point out, logic isn't concerned with meaning, which is its advantage because we can choose the analogues we want to use it for. It's like when McCarthy talked about submarines not being able to swim. They can't but we can pretend that they can pretend to (apologies to Bob B for the possible homomorphism, but even D&D overload the verb 'to know' when they talk about dbms's! As Djiksta said, when you get right down to it, all a program does is manipulate symbols. What the symbols mean is up to 'us').
But I would like to ask how is that a result can't be interpreted as an 'after value' when an algebraic equation does allow us to talk of a result of algebraic operations? What's more, no matter what changes happen over time, such an equation, if it doesn't contradict any others, as well as the result, is always true.
It is simple to invent algebraic operands that can be interpreted as 'before' and 'after', one just makes up names, eg., R' and R. The names can stand double-duty too in that (consistent) equations are invariant.
Personally I am coming to believe that the only "dire consequences" one can encounter turn up when one talks about the RM in terms of a language implementation, instead of sticking either to the algebra or to the calculus. Whether dire or not, I judge them to be self-inflicted consequences. Received on Wed Dec 17 2008 - 21:29:12 CET