Querying XML - 9.2 Performing Set Operations on Node Sets Using Value Semantics

Problem

You need to find the union, intersection, set difference, or symmetrical set difference between elements in two node sets; however, in your problem, equality is not defined as node-set identity. In other words, equality is a function of a node’s value.

Solution

XSLT 1.0

The need for this solution may arise when working with multiple documents. Consider two documents with the same DTD but content that may not contain duplicate element values. XSLT elements coming from distinct documents are distinct even if they contain elements with the same namespace, attribute, and text values. See Examples 9-1 to 9-4.

Example 9-1. people1.xslt

<people>
  <person name="Brad York" age="38" sex="m" smoker="yes"/>
  <person name="Charles Xavier" age="32" sex="m" smoker="no"/>
  <person name="David Williams" age="33" sex="m" smoker="no"/>
</people>

Example 9-2. people2.xslt

<people>
  <person name="Al Zehtooney" age="33" sex="m" smoker="no"/>
  <person name="Brad York" age="38" sex="m" smoker="yes"/>
  <person name="Charles Xavier" age="32" sex="m" smoker="no"/>
</people>

Example 9-3. Failed attempt to use XSLT union to select unique people

<xsl:template match="/">
  <people>
    <xsl:copy-of select="//person | document('people2.xml')//person"/>
  </people>
</xsl:template>

Example 9-4. Output when run with people1.xml as input

<people>
   <person name="Brad York" age="38" sex="m" smoker="yes"/>
   <person name="Charles Xavier" age="32" sex="m" smoker="no"/>
   <person name="David Williams" age="33" sex="m" smoker="no"/>
   <person name="Al Zehtooney" age="33" sex="m" smoker="no"/>
   <person name="Brad York" age="38" sex="m" smoker="yes"/>
   <person name="Charles Xavier" age="32" sex="m" smoker="no"/>
</people>

Relying on node identity can also break down in single document cases when you want equality of nodes to be a function of their text or attribute values.

Please check back next week for the continuation of this article.