Это называется " мюнхенская группировка ":
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<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:key name="kBooksByCat" match="book" use="category"/>
<xsl:template match="node()|@*">
<xsl:copy>
<xsl:apply-templates select="node()|@*"/>
</xsl:copy>
</xsl:template>
<xsl:template match=
"book
[generate-id()
=
generate-id(key('kBooksByCat', category)[1])
]
">
<category name="{category}">
<xsl:copy-of select="key('kBooksByCat', category)"/>
</category>
</xsl:template>
</xsl:stylesheet>
при применении к предоставленному документу XML :
<collection>
<book>
<title>Sushi for dummies</title>
<author>Judi Strada</author>
<category>Cooking</category>
<year>2001</year>
<isbn>95641022</isbn>
</book>
<book>
<title>Sixties Design</title>
<author>Philippe Garner</author>
<category>Design</category>
<year>2007</year>
<isbn>64781365</isbn>
</book>
<book>
<title>Final Jeopardy</title>
<author>Stephen Baker</author>
<category>Computer Science</category>
<year>2011</year>
<isbn>8316363546</isbn>
</book>
<book>
<title>Spoon river anthology</title>
<author>Edgar Lee Masters</author>
<category>Poetry</category>
<year>1973</year>
<isbn>21565648362</isbn>
</book>
<book>
<title>The dark is rising</title>
<author>Susan Cooper</author>
<category>Philosophy</category>
<year>1973</year>
<isbn>47884564151</isbn>
</book>
<book>
<title>The graphic alphabet</title>
<author>David Pelletier</author>
<category>Design</category>
<year>1996</year>
<isbn>1322456655</isbn>
</book>
<book>
<title>Pattern Recognition and Machine Learning</title>
<author>Christopher M. Bishop</author>
<category>Computer Science</category>
<year>2006</year>
<isbn>45456531073</isbn>
</book>
</collection>
желаемый, правильно сгруппированный результат получается :
<collection>
<category name="Cooking">
<book>
<title>Sushi for dummies</title>
<author>Judi Strada</author>
<category>Cooking</category>
<year>2001</year>
<isbn>95641022</isbn>
</book>
</category>
<category name="Design">
<book>
<title>Sixties Design</title>
<author>Philippe Garner</author>
<category>Design</category>
<year>2007</year>
<isbn>64781365</isbn>
</book>
<book>
<title>The graphic alphabet</title>
<author>David Pelletier</author>
<category>Design</category>
<year>1996</year>
<isbn>1322456655</isbn>
</book>
</category>
<category name="Computer Science">
<book>
<title>Final Jeopardy</title>
<author>Stephen Baker</author>
<category>Computer Science</category>
<year>2011</year>
<isbn>8316363546</isbn>
</book>
<book>
<title>Pattern Recognition and Machine Learning</title>
<author>Christopher M. Bishop</author>
<category>Computer Science</category>
<year>2006</year>
<isbn>45456531073</isbn>
</book>
</category>
<category name="Poetry">
<book>
<title>Spoon river anthology</title>
<author>Edgar Lee Masters</author>
<category>Poetry</category>
<year>1973</year>
<isbn>21565648362</isbn>
</book>
</category>
<category name="Philosophy">
<book>
<title>The dark is rising</title>
<author>Susan Cooper</author>
<category>Philosophy</category>
<year>1973</year>
<isbn>47884564151</isbn>
</book>
</category>
<book>
<title>The graphic alphabet</title>
<author>David Pelletier</author>
<category>Design</category>
<year>1996</year>
<isbn>1322456655</isbn>
</book>
<book>
<title>Pattern Recognition and Machine Learning</title>
<author>Christopher M. Bishop</author>
<category>Computer Science</category>
<year>2006</year>
<isbn>45456531073</isbn>
</book>
</collection>