Re: XQuery question

> As I understand it, the "relation" in relational theory concerns
> the relationship between the values of attributes in the *same* row
> (or tuple),

It seems to me the above definition of a relation is slightly differently than most texts. Most texts seem to say a relation is a set of related information having the structure of a header and a set of tuples where the named attributes can have values from their underlying domains. Most texts do not define a relation in terms of a tuple alone. Which is correct (according to rdm)?

> Relationships between values in different tables are captured via
> *integrity constraints*, of which referential integrity is just
> a special (albeit important) case.

So officially relationship is not a word defined by/in rdm. And relationship between values in different tables is captured via contraints. Thus in rdm-speak T_Car has a constraint with T_Color, which is good because otherwise people would get confused since the words relation and relationship are synonomous according to Roget's Thesaurus.

> What this means is that for example tables cannot have invisible
> pointers to save relationship information between different rows.
> All such references must be made via columns having the
> same value.

I think rdm says a tuple's attribute's value and the same value in the underlying domain have a contraint, but it does not specify how that constraint should be implemented, whether via equal values, IDs or even pointers, although experience thus far has us favoring the implementation with IDs. Does rdm infact specify which implmentation is proper and which is not?
If so, wouldn't that violate the principle of separation between logical and physical layers? Received on Wed Jun 04 2003 - 18:26:24 CEST