XSLT Cookbook

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

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 Example 7-1 to Example 7-4.

Example 7-1. 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 Willimas" age="33" sex="m" smoker="no"/>
</people>

Example 7-2. 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 7-3. 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 7-4. 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"/> ...