Re: On view updating

Paul wrote:

> mAsterdam wrote:
>

>> The interpretation of "X=2 OR X=3" as a constraint is:
>> Every assertion containing a value for X should have 2 or 3 as the 
>> value for X. It specifically does not assert that there should be 
>> assertions containing a value for X.


>  >

It is possible to express a constraint that would have "At least one of these statements is true: (1) 'X=2' (2) 'X=3'" as meaning, and the absence of both *would* constitute a violation - but we would need some more context.

> In a database that has no assertions about X (i.e. a relation exists but
> has no rows), if we ask: "Can X=2?" or if we ask: "Can X=3?", we will
> get the answer "No" both times.

Which interpreter are you using?

> Thus I would see the constraint being violated.

Yep.

> Maybe I'm not quite understanding the use of "X=2" though. I'm assuming
> this is shorthand for something like "there exists an employee with ID
> of 2". So you can have both X=2 and X=3 in the database at the same time
> with no contradiction. Unlike the mathematical sense where one would
> preclude the other (assuming 2 != 3 that is).
>
> Let's take a concrete SQL example.

Ah! Context.

> In one case you would have a table constraint like:
> EXISTS( SELECT * FROM emp WHERE id IN (2,3))
> In the other case you would have a row constraint like:
> id IN (2,3)
>
> Am I thinking of the former and you of the latter?

I was thinking of the latter.

Your statement (earlier post):
> just thought about this some more, and I realise now that
> the database can only contain these constraints "X=2 OR X=3"
> if one or the other of the statements is explicitly contained
> within the database. Because anything not in the database is
> assumed to be false.

needs an interpretation along the lines of the former.

> I think of the predicate and you of each individual proposition?
Could you elaborate, please? Received on Sun Sep 26 2004 - 17:38:05 CEST