Re: Another Schema Question

[ht@cogsci.ed.ac.uk (Henry S. Thompson)]

"Bullard, Claude L (Len)" <clbullar@ingr.com> writes:

> So"{fields} specifies XPath expressions relative to each element selected by
> a {selector}. This must identify a single node (element or attribute, not
> necessarily within the selected element) whose content or value, which must
> be of a simple type, is used in the constraint. It is possible to specify an
> ordered list of {fields}s, to cater to multi-field keys, keyrefs, and
> uniqueness constraints. "
> 
> That paragraph needs a bit more work.  So:  
> 
> "This must identify a single node whose content of value is 
> used in the contraint."  
> 
> Do all the parenthetical phrasing in a list or something.
> 
> So do fields specify a set of additional constraints or are these the
> complete constraint 
> set?  I am unclear as to the scope of the selector given that paragraph.

I'll discuss this as if it were just keys, keyref and unique are similar.

1) The key declaration establishes a scope, namely occurences of the
element in whose declaration it occurs;

2) The selector answers the question: "Within that scope, what
elements have keys?" -- answer: "every element which is selected by the 
selector XPath expression, starting at the scoping element."

3) The field(s) answer the question: "What uniquely identifies each
selected element?" -- answer: "The sequence of values of the field
XPath expressions, starting at the select4ed element".

Hope this helps

ht