Re: foreign key constraint versus referential integrity constraint

On Oct 25, 10:52 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> Marshall wrote:
> > On Oct 25, 4:15 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> >>Marshall wrote:
>
> >>>On Oct 24, 12:13 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> >>>>Domains, however, define what is or is not joinable because natural join
> >>>>applies the equality comparison to attributes with like names, and
> >>>>domains define equality comparisons.
>
> >>>That may be the most practical perspective, but from a theoretical
> >>>perspective, I find myself lately thinking that every value of every
> >>>domain is theoretically able to be compared for equality with
> >>>every value of every domain. If we are considering comparing
> >>>an integer and a string, we can say with confidence ahead of
> >>>time that the comparison will return that they are unequal.
>
> >>For the comparison, that's fine. But what is the type of the resulting
> >>attribute? We are discussing natural join.
>
> >>Suppose we have 2 relations:
>
> >>state: {
> >>      (id in char(2),name in string) |
> >>            ('AK','Alaska'), ('NY', 'New York) ...
> >>}
>
> >>inventory_bin: {
> >>      (id in integer, contents in string) |
> >>            ( 1, 'Soup' ), ( 2, 'Nuts' ) ...
> >>}
>
> >>When we evaluate state join inventory_bin, we know the body is empty,
> >>but what is the type of the id attribute?
>
> >>state join inventory_bin: {
> >>      (id in ??,name in string,contents in string) |
> >>}
>
> > Well. If the system is untyped, the question doesn't even
> > come up. (Not that I would ever use such a system, but
> > I mention it for completeness.)
>
> > In a typed system, the type is whatever the intersection
> > of int and string is.
>
> Union not intersection. It has to be a type compatible with both integer
> and string.

I don't see how that would work. An integer isn't type compatible with a string. Is it?

The other points you made seem to be variations on the same question.

> > On the weekends I often hang out with my oldest daughter at
> > the local Starbucks; I sip coffee and she people-watches. I
> > sit and she prefers to stand. If we go in the afternoon that
> > means I'm in the shade and she's not. Today we went at
> > twelve thirty, and she got sunburned.
>
> Ouch!

Fortunately, I'm not hot any more. Now I'm thinking that I haven't had any alcohol in a week or more. Maybe a drink is in order? Plus, I think there is a cupcake in a cupboard somewhere downstairs. I must away!

Marshall Received on Tue Oct 27 2009 - 05:52:22 CET