Re: Normalisation

Jon Heggland wrote:
> 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.

http://en.wikipedia.org/wiki/Class_(set_theory)

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

Because by definition it isn't, and redefining the notion of domain such that it is, is not that easy without either running into paradoxes or getting a notion which it is almost impossible to reason about.

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

I am not disallowing anything.

• Jan Hidders