Group/merge childs of same nodes in xml/xslt

Question

I am new to XSLT and changing it manually will take a lot of time.

<GroupData ID="xxx" Key="4" Temp="yyy">
 <ItemData ID="zzz" Value="3"/>
</GroupData>

<GroupData ID="xxx" Key="4" Temp="yyy">
 <ItemData ID="www" Value="1982"/>
</GroupData>

I want to have the childs of these multiple GroupData nodes within the same group, i.e.,

<GroupData ID="xxx" Key="4" Temp="yyy">
 <ItemData ID="zzz" Value="3"/>
 <ItemData ID="www" Value="1982"/>
</GroupData>

So I need to merge/combine/match them on both GroupData's ID and Key attributes (these vary within the file). Also some do not have a Key attribute. How can I do that? I read some other threads (for example, in C# but I don't have that at my disposal) and I checked W3 schools but these are very basic examples. I am using the latest XML Tools 2.3.2 r908 unicode (beta4) for Notepad++ to apply possible transformations (do not know whether it supports XSLT2.0 or XSLT1.0).

Edit: After trying the suggestions below and various things I am stuck since it has multiple levels and possiblly does not have unique IDs: ...

Answer 1

If it is XSLT 2.0 then you can use a nested <xsl:for-each-group>

<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <Groups>
      <xsl:for-each-group select="/Groups/GroupData" group-by="@ID">
        <xsl:for-each-group select="current-group()" group-by="if(@Key) then @Key else 'no key'">
          <GroupData>
            <!-- Copy attributes off the *first* GroupData element in the group -->
            <xsl:copy-of select="current-group()[1]/@*"/>
            <!-- Copy ItemData children from *all* GroupData elements in the group -->
            <xsl:copy-of select="current-group()/ItemData" />
          </GroupData>
        </xsl:for-each-group>
      </xsl:for-each-group>
    </Groups>
  </xsl:template>
</xsl:stylesheet>

(I'm assuming your input file has a root element <Groups> and uses no namespaces).

If it's XSLT 1.0 then you need to use Muenchian Grouping:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:key name="group-data" match="GroupData" use="concat(@ID, '___', @Key)" />
  <xsl:template match="/">
    <Groups>
      <!--
      Iterate over a node set containing just one GroupData element for
      each combination of ID and Key
      -->
      <xsl:for-each select="/Groups/GroupData[count( . | key('group-data', concat(@ID, '___', @Key))[1]) = 1]">
        <GroupData>
          <!-- Copy attributes from the "prototype" GroupData -->
          <xsl:copy-of select="@*"/>
          <!--
          Copy ItemData children from *all* GroupData elements with matching
          ID/Key
          -->
          <xsl:copy-of select="key('group-data', concat(@ID, '___', @Key))/ItemData" />
        </GroupData>
      </xsl:for-each>
    </Groups>
  </xsl:template>
</xsl:stylesheet>

Here I'm doing a single grouping pass based on both the ID and Key attributes by creating a synthetic key value of {ID}___{Key}.

Answer 2

This XSLT 1.0 transformation:

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>
 <xsl:strip-space elements="*"/>

 <xsl:key name="kGDByIdKey" match="GroupData"
  use="concat(@ID, '+', @Key)"/>

 <xsl:template match="node()|@*">
     <xsl:copy>
       <xsl:apply-templates select="node()|@*"/>
     </xsl:copy>
 </xsl:template>

 <xsl:template match=
 "GroupData
    [generate-id()
    =
     generate-id(key('kGDByIdKey', concat(@ID, '+', @Key))[1])
     ]">
    <xsl:copy>
      <xsl:apply-templates select=
       "@*|key('kGDByIdKey', concat(@ID, '+', @Key))/node()"/>
    </xsl:copy>
 </xsl:template>

  <xsl:template match="GroupData"/>
</xsl:stylesheet>

when applied on this XML document:

<t>
    <GroupData ID="xxx" Key="4" Temp="yyy">
        <ItemData ID="zzz" Value="3"/>
    </GroupData>
    <GroupData ID="yyy" Key="4" Temp="yyy">
        <ItemData ID="abc" Value="3"/>
    </GroupData>
    <GroupData ID="zzz" Temp="yyy">
        <ItemData ID="pqr" Value="1982"/>
    </GroupData>
    <GroupData ID="xxx" Key="4" Temp="yyy">
        <ItemData ID="www" Value="1982"/>
    </GroupData>
    <GroupData ID="yyy" Key="4" Temp="yyy">
        <ItemData ID="def" Value="1982"/>
    </GroupData>
    <GroupData ID="zzz" Temp="yyy">
        <ItemData ID="tuv" Value="1982"/>
    </GroupData>
</t>

produces the wanted, correct result:

<t>
   <GroupData ID="xxx" Key="4" Temp="yyy">
      <ItemData ID="zzz" Value="3"/>
      <ItemData ID="www" Value="1982"/>
   </GroupData>
   <GroupData ID="yyy" Key="4" Temp="yyy">
      <ItemData ID="abc" Value="3"/>
      <ItemData ID="def" Value="1982"/>
   </GroupData>
   <GroupData ID="zzz" Temp="yyy">
      <ItemData ID="pqr" Value="1982"/>
      <ItemData ID="tuv" Value="1982"/>
   </GroupData>
</t>

Explanation:

Proper use of the Muenchian grouping method and the identity rule.