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The keyref Element

The keyref identity constraint assures that each key sequence specified by the keyref exists, by virtue of equality, as a key sequence in the reference identity constraint. The reference identity constraint is always a unique or key identity constraint.

The following example specifies that every item in an order must exist in the list specified by the already specified partNumberKey:

<xsd:keyref name="partNumberRef"
    <xsd:documentation xml:lang="en">
      Each order must refer to a known part number.
  <xsd:selector xpath="compressedOrder/order/item"/>
  <xsd:field xpath="partNumber"/>

Note that while this article does not provide examples (the book does!), a unique, key, and keyref each might specify multiple fields (that is, a multi-valued key such as "last name" and "first name" combined).

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