Groups two xml files like a sql group-by [2] - xslt

Question

This is an evolution of my ask here :
Groups two xml files like a sql group-by
The Example given and Dimitre Solution was counting distinct isbn value.
Now modify library xml to have
mylibrary.xml :
<library>
<book id="1" isbn="1"/>
<book id="2" isbn="1"/>
<book id="3" isbn="2"/>
<book id="4" isbn="4"/>
<book id="5" isbn="5"/>
<book id="6" isbn="4"/>
<book id="7" isbn="4"/>
</library>
and this one that can be used :
bookreference.xml :
<reference>
<book isbn="1">
<category>SF</category>
</book>
<book isbn="2">
<category>SF</category>
</book>
<book isbn="3">
<category>SF</category>
</book>
<book isbn="4">
<category>Comedy</category>
</book>
<book isbn="5">
<category>Comedy</category>
</book>
</reference>
i want to get the numbers of book i got in mylibrary 'even if some have same isbn', groupby category, using xslt 1-0.
output wanted:
SF : 3 book(s)
Comedy : 4 book(s)
my xslt propose here : Groups two xml files like a sql group-by works fine but of course use 'for-each' loop and extension functions.
Surely there is a better solution.

Answer 1

Again a very good question! (+1)
This transformation, using two keys for achieving full efficiency:
<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="kBookByCat" match="book"
use="category"/>
<xsl:key name="kBookByIsbn" match="book"
use="#isbn"/>
<xsl:variable name="vDoc" select="/"/>
<xsl:variable name="vRef" select=
"document('file:///c:/temp/delete/reference.xml')"/>
<xsl:variable name="vMyIsbns" select="/*/*/#isbn"/>
<xsl:variable name="vResult">
<xsl:apply-templates select="$vRef/*"/>
</xsl:variable>
<xsl:template match="/">
<xsl:copy-of select="$vResult"/>
</xsl:template>
<xsl:template match=
"book[generate-id()
=
generate-id(key('kBookByCat', category)[1])
]
">
<xsl:variable name="vBooksinCat" select=
"key('kBookByCat', category)"/>
<xsl:value-of select="category"/> : <xsl:text/>
<xsl:for-each select="$vDoc">
<xsl:value-of select="count(key('kBookByIsbn',$vBooksinCat/#isbn))"/>
</xsl:for-each>
<xsl:text> book(s)
</xsl:text>
</xsl:template>
<xsl:template match="text()"/>
</xsl:stylesheet>
when applied on the provided XML document contained in the file mylibrary.xml:
<library>
<book id="1" isbn="1"/>
<book id="2" isbn="1"/>
<book id="3" isbn="2"/>
<book id="4" isbn="4"/>
<book id="5" isbn="5"/>
<book id="6" isbn="4"/>
<book id="7" isbn="4"/>
</library>
and having this provided XML document in C:\temp\delete\reference.xml:
<reference>
<book isbn="1">
<category>SF</category>
</book>
<book isbn="2">
<category>SF</category>
</book>
<book isbn="3">
<category>SF</category>
</book>
<book isbn="4">
<category>Comedy</category>
</book>
<book isbn="5">
<category>Comedy</category>
</book>
</reference>
produces the wanted, correct output:
SF : 3 book(s)
Comedy : 4 book(s)

Answer 2

modified Dimitri version to work for this
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:key name="kBookByCat" match="book" use="category"/>
<xsl:variable name="vRef" select="document('file:///c:/temp/delete/reference.xml')"/>
<xsl:variable name="meh" select="*"/>
<xsl:template match="/">
<xsl:apply-templates select="$vRef/reference/book[generate-id()=generate-id(key('kBookByCat', category)[1])]" />
</xsl:template>
<xsl:template match="book">
<xsl:variable name="cat" select="category"/>
<xsl:value-of select="category"/> : <xsl:text/>
<xsl:variable name="isbns" select="$vRef/reference/book[category=$cat]/#isbn"/>
<xsl:value-of select="count($meh/book[#isbn=$isbns])"/>
<xsl:text> book(s)
</xsl:text>
</xsl:template>

Answer 3

in add to the great dimitri response, i propose the following to not print book category that have 0 book set in mylibrary :
<xsl:variable name="catname" select="category"/>
<xsl:for-each select="$vDoc">
<xsl:variable name="cnt" select="count(key('kBookByIsbn',$vBooksinCat/#isbn))"/>
<xsl:if test="$cnt > 0">
<xsl:value-of select="$catname"/> :
<xsl:text/>
<xsl:value-of select="$cnt"/>
<xsl:text> book(s)
</xsl:text>
</xsl:if>
</xsl:for-each>