Re: NULLs: theoretical problems?

On 17 aug, 00:03, Hugo Kornelis <h..._at_perFact.REMOVETHIS.info.INVALID> wrote:
>
> [....big snip ...]
>
> And the model is extremely unstable, since it's not uncommon in real
> businesses to see an optional proposition become mandatory (e.g. ebcause
> of legislation change) or a mandatory proposition become optional (e.g.
> because of competition). If all propositions are combined into a single
> table, this is a simple change from NULL to NOT NULL or vice versa. If
> each proposition has its own table, this is an equally simple change,
> the addition or removal of a constraint that SQL does not currently
> support but that is known as "equality constraint" in Object Role
> Modeling.

Just a minor technicality. As far as I understand it an equality constraint can be represented by two inclusion dependencies in both directions, so I would say that it *is* supported by SQL. Anything I'm missing?

Btw. your interpretation of null values is basically the "missing value" interpretation, with which theoretically speaking, there is of course no problem whatsoever. But your insistence that NULL somehow at the same time a marker that indicates that a value is missing and is also a value itself (because an element of the domain) is a little bit suspect. There are better and more consistent ways of describing missing values.

• Jan Hidders