Re: Normalisation

In article <AyVye.138732$g63.7370802_at_phobos.telenet-ops.be>, jan.hidders_at_REMOVETHIS.pandora.be says...
> >>Note btw. that if user-defined functions are restricted to the
> >>domains (their input and output types are only domains) then you cannot
> >>define such an operation as a user-defined function.
> >
> > Let me try to define such an operator.
> >
> > unnest_string takes a relation IN and an attribute name A as arguments,
> > returns a relation OUT. The type of attribute A in IN is character
> > string.
>
> Ah, but now you are using the domain or relations, right? There is a
> problem with that domain. It doesn't exist. The collection of all
> relations is a proper class, and not a set, but domains have to be sets.

You'll have to educate me on the difference between "proper class" and "domain", I'm afraid. The term "class" is used for so many slightly different things.

Be that as it may, what is the problem?

Should I be forbidden from treating "relation" as a (generic) domain when defining this operator? Why?

Would you allow this operator if it were system-defined?

-- 
Jon