Re: [LIU Comp Sci] Need tutoring on Relational Calculus
Date: Mon, 22 Dec 2014 03:14:43 -0500
Message-ID: <m78jtj$23a$1_at_reader1.panix.com>
On 12/21/2014 08:26 AM, Eric wrote:
> Good point. Though the "universe" of a database is only those things
> that it is designed to store, not the star dust and dark matter. Free
> and bound here are merely relative to that "universe of discourse".
the universe of discourse....
OK that wasn't in the text.
"In the formal sciences, the domain of discourse, also called the universe of discourse (or simply universe), is the set of entities over which certain variables of interest in some formal treatment may range."
See and this is exactly what is challenging me. Everything is bound. Nothing is free :(
"Before you construct the actual query (or formula) the tuple variable is free to have any value - you don't know what the answer will be. Afterwards, because of what you put in the formula, the tuple variable will be bound - to the answer you need!"
Other that the data's generalized characteristic, I don't know its specific sub-domain according to some logical rule set.
- In the formula F, the variable t is not bound, it could take any, all, or none of the values possible in the universe of discourse.
- F' = (∃ t)(F) is a different formula and in this formula t is bound
because "none" is no longer allowed.
Can we dissect this syntax.
F' is equal to all the tuples (t) that exist when the rules of F are applied to t?
QUOTE:What other quantifiers would you have? And what's wrong with
universal?
(∀ t)(F) just says that F is a fact, true for all values of the variable
t.
Why all? Why not half? Aside from which it seems non-sensible to me. It is a condition that can never be met. There is never a time that ALL tuples match a logical construction. That would be a non restraint and it would seem to be devoid of information. ALL possible answers are TRUE. It sounds like divorce court actually. Received on Mon Dec 22 2014 - 09:14:43 CET