这个样式表...
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" indent="yes"/>
<xsl:key name="groups" match="ProjectGroup" use="GroupID" />
<xsl:template match="@*|node()">
 <xsl:copy> 
   <xsl:apply-templates select="@*|node()" />
 </xsl:copy> 
</xsl:template>
<xsl:template match="ProjectGroup[generate-id()=generate-id(key('groups',GroupID)[1])]">
 <xsl:copy> 
   <xsl:apply-templates select="@*" />
   <xsl:attribute name="count-of-projects">
     <xsl:value-of select="count(key('groups',GroupID))" />
   </xsl:attribute>
   <xsl:apply-templates select="node()" />
 </xsl:copy> 
</xsl:template>
</xsl:stylesheet>
...当应用于您的输入时,将产生...
<?xml version="1.0" encoding="utf-8"?>
<ProjectGroups>
    <ProjectGroup count-of-projects="1">
        <GroupID>1</GroupID>
        <ProjectName>Project 1</ProjectName>
    </ProjectGroup>
    <ProjectGroup count-of-projects="2">
        <GroupID>2</GroupID>
        <ProjectName>Project 2</ProjectName>
    </ProjectGroup>
    <ProjectGroup>
        <GroupID>2</GroupID>
        <ProjectName>Project 3</ProjectName>
    </ProjectGroup>
</ProjectGroups>