XSLT: Do a group by twice, first within the same t

Question

I want to group the following XML:

<DataSet>
  <FirstNode>
    <UniqueKey>111</UniqueKey> 
    <OtherKey>552</OtherKey>
  </FirstNode>
  <FirstNode> 
    <UniqueKey>123</UniqueKey> 
    <OtherKey>552</OtherKey>
  </FirstNode>
  <FirstNode> 
    <UniqueKey>154</UniqueKey> 
    <OtherKey>553</OtherKey>
  </FirstNode>
  <SecondNode>
    <FirstNodeKey>111</FirstNodeKey>
  </SecondNode>
  <SecondNode>
    <FirstNodeKey>123></FirstNodeKey>
  </SecondNode>
  <SecondNode>
    <FirstNodeKey>154></FirstNodeKey>
  </SecondNode>
</DataSet>

I want to produce the following xml with XSLT:

  <DataSet>
      <FirstNode>
        <UniqueKey>111</UniqueKey> 
        <OtherKey>552</OtherKey>
      </FirstNode>
      <FirstNode> 
        <UniqueKey>123</UniqueKey> 
        <OtherKey>552</OtherKey>
      </FirstNode>
      <SecondNode>
        <FirstNodeKey>111</FirstNodeKey>
      </SecondNode>
      <SecondNode>
        <FirstNodeKey>123></FirstNodeKey>
      </SecondNode>
 </DataSet>

<DataSet>
      <FirstNode> 
        <UniqueKey>154</UniqueKey> 
        <OtherKey>553</OtherKey>
      </FirstNode>
      <SecondNode>
        <FirstNodeKey>154></FirstNodeKey>
      </SecondNode>
 </DataSet>

Basically I want to group the FirstNodes by the OtherKey first, and then group by the UniqueKey and FirstNodeKey. Then each should be enclosed in <DataSet></DataSet>. Can I do this by using grouping?

Thanks in advance for your help!

Answer 1

It seems you simply want to group the FirstNode elements by the OtherKey child and then reference any SecondNode elements based on current-group()/UniqueKey:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    exclude-result-prefixes="#all"
    version="3.0">

  <xsl:mode on-no-match="shallow-copy"/>

  <xsl:output method="xml" indent="yes"/>

  <xsl:key name="second" match="SecondNode" use="FirstNodeKey"/>

  <xsl:template match="DataSet">
     <xsl:variable name="ds" select="."/>
     <xsl:for-each-group select="FirstNode" group-by="OtherKey">
         <xsl:copy select="$ds">
             <xsl:copy-of select="current-group(), key('second', current-group()/UniqueKey, .)"/>
         </xsl:copy>
     </xsl:for-each-group>
  </xsl:template>

</xsl:stylesheet>

That is XSLT 3 working with Saxon 9.8 (example at https://xsltfiddle.liberty-development.net/3NzcBtw) or Altova 2018, for XSLT 2 you could spell out the

         <xsl:copy select="$ds">
             <xsl:copy-of select="current-group(), key('second', current-group()/UniqueKey, .)"/>
         </xsl:copy>

as

<DataSet> 
     <xsl:copy-of select="current-group(), key('second', current-group()/UniqueKey, $ds)"/>
</DataSet>

and of course if there are other nodes to process replace the xsl:mode declaration with the identity template.