Re: Another view on analysis and ER

Sorry for butting in this late, and not even completely on topic. 'Facts' triggered my interest.

Jon Heggland schreef:

> ...(The idea of viewing a database as a
> collection of facts was a revelation for me in that regard.) In fact, I
> have the opposite problem; I am unable to look at an E/R diagram without
> thinking about relations.
>
> Consider this proposition: "Jon was born in 1974", encoded in a relvar
> of the form Born(Person, Year). I think we'll agree that represents a
> fact about Jon. You would probably assume that Jon is an entity (though
> I'm unsure about what you'd call the relvar/predicate in itself---is it
> an entity (type)?). But I would also say that the proposition is as much
> a fact about the year 1974! Is 1974 an entity? I really don't care.
> Facts are all.

Consider the statement "Jon is 33 years old". It conveys the same real world fact in a clumsier way than "Jon was born in 1974". Next year it won't even convey the same fact anymore. "Jon was born in 1974" catches the invariant better than "Jon is 33 years old".

Consider "John is in Canada". When? The fact isn't complete without that piece of information.

"<Person> is at <Location>" needs a time to become a possibly interesting facttype:
"At <Time>, <Person> is/was/will be at <Location>"

The idea of viewing a database as a collection of possibly interesting facts was a revelation for me :-)

[snip]

> A propositional encoding does specify how this marriage is identified,
> yes. What I dispute is the distinction between implicit and explicit
> identification; between entities and relationship. A tuple/fact is
> identified by some combination of its attributes, that is all.

minus "that is all": agreed. The choice of the combination of attributes takes care, some of which is specific to the facttype (was_born vs. has_age), and some of which is more general (At time: ...). Received on Fri Dec 07 2007 - 13:55:41 CET