Re: NULLs: theoretical problems?
Date: Fri, 17 Aug 2007 10:00:29 -0700
On Aug 17, 2:17 pm, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "Jan Hidders" <hidd..._at_gmail.com> wrote in message
> > 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?
> Yes. SQL doesn't support multiple assignment. In order to get rows with
> new key values inserted, at least one of the constraints needs to be
Or you use deferrable constraints. Present since SQL-92 and supported by most serious DBMSs, AFAIK.
- Jan Hidders