Re: Codd's Information Principle
Date: Sat, 31 Oct 2009 20:03:30 GMT
Bob Badour wrote:
> paul c wrote:
>> I was quoting Mr. Scott. Regardless, I don't agree with either
>> interpretation. I realize that many, perhaps most, people who have
>> been trained in logic or read about it would place my attitude
>> somewhere between unfaithful and ignorant, but I would never try to
>> tell a user that some predicates are conjunctions and some aren't.
> Even if the user asked? Is it like taboo or something?
Heh, not taboo in a fatal sense, just pointless and possibly a wasteful distraction.
Distinguishing between everyday 'and' and logical 'AND' in user relations, eg., saying part 'P1' is available in London from supplier 'S1' versus saying supplier 'S1' is in London AND supplier 'S1' supplies part 'P1' is probably harmless. But I would not know how to answer the user who then asked "where is the 'AND' in the relation's value?", except to ask them back "well, what is the difference?".
If a join was involved to produce such a relation, a designer might find one statement a more helpful reminder of his own actions than the other but the extension's value is what matters to users (not to mention references to the value by other operations). The answer 'fifteen dollars owed' doesn't reflect whether it is a sum or a difference or some other calculation. A user who wants to know which customers have credits doesn't expect the receivables balance to answer the question.
Personally I would give users the simpler predicate, just as I would not feel obliged to tell them that the answer to '6 + 9' involved a carry operation. Received on Sat Oct 31 2009 - 21:03:30 CET