Re: Normalisation

In article <ZaSxe.136291$Ql.7257205_at_phobos.telenet-ops.be>, jan.hidders_at_REMOVETHIS.pandora.be says...
> > How do you define atomicity?
>
> Roughly, I would say that it means that all your data manipulation can
> be expressed as operations over a signature that consists only of (1)
> domains described by an abstract data type and (2) relations that
> contain in their fields only values from these domains.

I can't make heads or tails of this. Operations over a signature? Domains described by an abstract data type? Is a domain not the same as a data type? Can a data type describe several domains? And again with the "abstract"---does your atomicity require that all data types be abstract? What, then, is a concrete (or non-abstract) data type, and why is it not allowed? Are relation types not (abstract) data types? How about set types?

> An unnest operation, for example, cannot be described in that way.

What if I define an unnest operator that "unnests" strings in a corresponding manner?

Anyway, let me rephrase my initial question. What is an atomic datatype/domain?
What is a non-atomic datatype/domain?
What bad happens if you allow a non-atomic datatype in a relation?

-- 
Jon