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

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)&#xA;</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)