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Scope
Each identity constraint identifies a set of nodes that must be unique or that require referential integrity. Each identity constraint has a specific scope, which is the enclosing element type; any element type may provide any number of identity constraints.
In the example schema, the following schema excerpt specifies a key relative to the element type idConstraintDemo:
<xsd:element name= "idConstraintDemo">
<xsd:complexType>
* * * *
</xsd:complexType>
* * * *
<xsd:key name="partNumberKey">
<xsd:annotation>
<xsd:documentation xml:lang="en">
The part number uniquely identifies
each orderable item.
</xsd:documentation>
</xsd:annotation>
<xsd:selector xpath="catalog/*"/>
<xsd:field xpath="partNumber"/>
</xsd:key>
* * * *
</xsd:element>
The example for this book was written awhile ago: in retrospect, the catalog should be a standalone element type in its own namespace (a good exercise!). Regardless, the existing schema demonstrates not only this key, but others that have the same scope as idConstraintDemo.