Re: NULLs: theoretical problems?
Date: Fri, 17 Aug 2007 12:17:21 GMT
"Jan Hidders" <hidders_at_gmail.com> wrote in message
> On 17 aug, 00:03, Hugo Kornelis <h..._at_perFact.REMOVETHIS.info.INVALID>
>> [....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
> 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
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 disabled.
> 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
Received on Fri Aug 17 2007 - 14:17:21 CEST