Re: Constraints and Functional Dependencies

Marshall wrote:
> ...
> Hmmm. Can we express keyness otherwise? I can't think how.
> ...

I've long imagined that Codd would have defined it (as you also did) via a cartesian product but some others like to use cardinality, eg., adding a "count" operation to their theory. My guess is that they see this as something many implementations would want anyway.

I can't guess whether Codd would have also required a RENAME operator or whether he would have stuck with the math approach of numbering attributes for explanatory purposes. Then there is projection, always lurking in the background, and we can't ignore it without breaking who knows what. I sometimes wonder whether an alternative concept or two, such as a variation on D&D's GROUP/UNGROUP operators might allow definition of keys without rename or your prime operator. By itself I doubt whether there would be any practical point to examining that unless some other useful questions could be handled as well, perhaps deciding when two relations that involve seemingly different rva's are equivalent but I don't have anything actually useful to say about that.

I'm going to go look again, maybe Codd used cardinality and I've just forgotten about it.

p Received on Sun Feb 25 2007 - 18:56:39 CET