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Subject: Grouping and Summaries  in XSLT
To: xalan-dev@xml.apache.org
Cc: cocoon-dev@xml.apache.org
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Over time I've been asked a lot about how to do grouping and summaries in
XSLT, and this came up the other day again, and I thought I would share the
results.  This is better a subject for the xsl list, but I thought it would
be useful to post it to the xalan list and the cocoon list.

You can do anything with XSLT, if you're warped enough.  That said, XSLT is
not particularly good at grouping at this time (this is one of the things
that will go into XSLT V2).

The only technique I know of to group without knowing the selection
criteria of the groups ahead of time, is to recurse into a named template
for each element passing in a string, add the found selection criteria to
the string via the concat function, and then test with the contains()
function if the selection criteria exists withing the string.  It's ugly
and a bit slow, but it works.  (This would be easy if you had for loops and
assignable variables in XSLT, but such is the design constraints of the
language...)

In the sample below, the user wanted to group the rows by server, and then
provide summaries for each month's memory consumption.   The transformation
is a useful example of how to do two-level grouping using the above
technique.  It would be nice to paramiterise the templates and simply put
them in a library, but that would be difficult, I think.  I'm sure the
named templates could be improved in a couple of ways for better
performance.  I'll be interested to see what ideas folks come up with.
BTW, you could also use extensions to do this, but the transformation below
is totally interoperable.

The input data:
<TABLE >
   <ROW>
       <fqhn  Value="pebbles"/>
       <mem_pct_used  Value="54"/>
       <date Value="20/02/00"/>
   </ROW>
   <ROW>
       <fqhn  Value="pebbles"/>
       <mem_pct_used  Value="11"/>
       <date  Value="21/02/00"/>
   </ROW>
   <ROW >
       <fqhn  Value="bambam"/>
       <mem_pct_used  Value="27"/>
       <date  Value="10/03/00"/>
   </ROW>
   <ROW >
       <fqhn  Value="pebbles"/>
       <mem_pct_used  Value="22"/>
       <date  Value="10/03/00"/>
   </ROW>
</TABLE>

And a stylsheet:
<?xml version="1.0"?>
<xsl:stylesheet
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="1.0">

  <xsl:output indent="yes"/>

  <!-- Named recursive template that finds groups by fqhn and month -->
  <xsl:template name="group-by-fqhn-and-month">
    <xsl:param name="found-names"/>
    <xsl:param name="row-position"/>
    <xsl:param name="fqhn"/>

    <!-- if the string does not contain our fqhn/@Value, we found a first
match -->
    <xsl:for-each select="ROW[$row-position][fqhn/@Value=$fqhn]">
      <xsl:variable name="date-value" select="substring(date/@Value, 4,
2)"/>
      <xsl:if test="not(contains($found-names, $date-value))">
        <xsl:variable name="date-group" select="/TABLE/ROW[fqhn/@Value
=$fqhn and substring(date/@Value, 4, 2)=$date-value]"/>
        <Server_Performance_Info Month="{$date-value}/00" Sum="{sum
($date-group/mem_pct_used/@Value)}" />
      </xsl:if>
    </xsl:for-each>

    <!-- recurse to the next position, passing in the fqhn/@Value
concatenated to
         the found-names variable -->
    <xsl:if test="ROW[$row-position+1]">
      <xsl:variable name="month" select="substring(ROW
[$row-position][fqhn/@Value=$fqhn]/date/@Value, 4, 2)"/>
      <xsl:call-template name="group-by-fqhn-and-month">
        <xsl:with-param name="fqhn" select="$fqhn"/>
        <xsl:with-param name="row-position" select="$row-position+1"/>
        <xsl:with-param name="found-names" select="concat($found-names,
$month, '#')"/>
      </xsl:call-template>
    </xsl:if>
  </xsl:template>


  <!-- Named recursive template that finds groups by fqhn -->
  <xsl:template name="group-by-fqhn">
    <xsl:param name="found-names"/>
    <xsl:param name="row-position"/>

    <!-- if the string does not contain our fqhn/@Value, we found a first
match -->
    <xsl:for-each select="ROW[$row-position]">
      <xsl:if test="not(contains($found-names, fqhn/@Value))">
        <Server>
          <xsl:variable name="fqhn-value" select="fqhn/@Value"/>
          <Server_Configuration_Info  Server_id="{$fqhn-value}"/>
          <!-- now select the rows that match our date -->
          <xsl:for-each select="/TABLE">
            <xsl:call-template name="group-by-fqhn-and-month">
              <xsl:with-param name="fqhn" select="$fqhn-value"/>
              <xsl:with-param name="row-position" select="1"/>
              <xsl:with-param name="found-names" select="''"/>
            </xsl:call-template>
          </xsl:for-each>
        </Server>
      </xsl:if>
    </xsl:for-each>

    <!-- recurse to the next position, passing in the fqhn/@Value
concatenated to
         the found-names variable -->
    <xsl:if test="ROW[$row-position+1]">
      <xsl:call-template name="group-by-fqhn">
        <xsl:with-param name="row-position" select="$row-position+1"/>
        <xsl:with-param name="found-names" select="concat($found-names, ROW
[$row-position]/fqhn/@Value, '#')"/>
      </xsl:call-template>
    </xsl:if>
  </xsl:template>

  <xsl:template match="/">
    <RESULT>
    <xsl:for-each select="TABLE">
      <xsl:call-template name="group-by-fqhn">
        <xsl:with-param name="row-position" select="1"/>
        <xsl:with-param name="found-names" select="''"/>
      </xsl:call-template>
    </xsl:for-each>
    </RESULT>
  </xsl:template>

</xsl:stylesheet>

will give you:

<?xml version="1.0" encoding="UTF-8"?>
<RESULT>
    <Server>
        <Server_Configuration_Info Server_id="pebbles"/>
        <Server_Performance_Info Sum="65" Month="02/00"/>
        <Server_Performance_Info Sum="22" Month="03/00"/>
    </Server>
    <Server>
        <Server_Configuration_Info Server_id="bambam"/>
        <Server_Performance_Info Sum="27" Month="03/00"/>
    </Server>
</RESULT>

-scott