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Re: Another Schema Question

• From: ht@c... (Henry S. Thompson)
• To: "Bullard, Claude L (Len)" <clbullar@i...>
• Date: Tue, 19 Jun 2001 08:46:18 +0100

"Bullard, Claude L (Len)" <clbullar@i...> 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

• References:
  • Another Schema Question
    • From: "Bullard, Claude L (Len)" <clbullar@i...>

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