XML in a Nutshell, 3rd Edition

Name

xs:selector

Synopsis

<xs:selector
  id = "ID"
  xpath = "XPath expression" >
  <!-- (xs:annotation?) -->
</xs:selector>

A single xs:selector element is placed inside each xs:unique, xs:key, and xs:keyref element to specify the element nodes for which the key or key reference is defined. The node set is selected by an XPath expression contained in the value of the xpath attribute. The context node for this XPath expression is the element matched by the xs:element declaration in which the xs:unique, xs:key, or xs:keyref element appears.

Not all XPath expressions are allowed here. In particular, the XPath expression must be an abbreviated location path that limits itself to the child axis. The only node tests used are element name, the * wildcard, and the prefix :* wildcard. Abbreviated syntax must be used; predicates are not allowed. Thus, person/name/first_name is a legal XPath expression for this attribute, but person//name and name/first_name/@id are not. Several instances of this restricted form of XPath expression can be combined with the vertical bar so that person/name/first_name | person/name/last_name is also an acceptable XPath expression. Finally, the XPath expression may begin with .// so that .//name is valid. However, this is the only place the descendant-or-self axis can be used; no other forms of XPath expression are allowed here.