Re: Thinking about MINUS

On Jan 7, 8:48 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> paul c wrote:
>
> > One of a couple of reasons this topic intrigues me is that certain
> > scenarios aren't closed for operations in Codd's framework, eg., the
> > case when two operands share an attribute name that has different types.
> > I realize that a whole sub-industry has been built to deal with
> > problems like this (based on various design disciplines, knowingly or
> > unknowingly, I don't know), so many people would say I'm silly to
> > wonder, but I can't help it.
>
> Wouldn't one end up with a resulting attribute defined as a union-type
> in a relation with no rows?

That's certainly one way to handle it. However I would be more inclined to think of it as a type error. Considering the relational operations as
logic operations, if we use a variable to mean one thing (or kind of thing)
in one equation, and another thing in another equation, we can't validly
proceed using both at once.

Marshall Received on Sun Jan 07 2007 - 21:00:34 CET