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Simple XSLT&XPath
grouping
Teemu Keiski
I
have lately seen questions on ASP.NET Forums how to utilize grouping or selecting
distinct nodes from XML document using XSLT or plain XPath. In this article I
show couple of simple examples of such functionality.
Sample data
Sample XML
data that both examples utilize is as follows.
<?xml version="1.0" encoding="utf-8" ?>
<cars>
<car manufacturer="Mazda" owner="Jason" />
<car manufacturer="Opel" owner="Teemu" />
<car manufacturer="Toyota" owner="Thomas" />
<car manufacturer="Opel" owner="Henry" />
<car manufacturer="Daimler" owner="Mika" />
<car manufacturer="Toyota" owner="Colt" />
<car manufacturer="Toyota" owner="Teemu" />
<car manufacturer="Daimler" owner="Jason" />
</cars>
Selecting distinct nodes
How to
select distinct manufacturers from the sample data? Well, like this for
example:
//car[not(@manufacturer=preceding-sibling::car/@manufacturer)]/@manufacturer
How it
works? It drops any car element that have same manufacturer attribute value
that preceding sibling car element has (if such value exists in other words).
Grouping by manufacturers in XSLT stylesheet
In this scenario
selecting distinct manufacturer nodes is the key thing, although logic is bit
more complicated. Distinct nodes are
first selected into variable
<xsl:variable name="uniquemanufacturers" select="//car[not(@manufacturer=preceding-sibling::car/@manufacturer)]/@manufacturer" />
Then
manufacturers are looped through using <xsl:for-each> and also in sorted
order. For each manufacturer, car owners are queries as follows in the loop.
<xsl:variable name="owners" select="//car[@manufacturer=current()]/@owner" />
current()
function refers to the current (context) node when for-each loop processing
goes node by node. After this count of owners is printed out using
<xsl:value-of select="count($owners)" />
And finally
car owners are listed in sorted order simply
<xsl:for-each select="$owners">
<xsl:sort select="." />
<xsl:value-of select="current()" /><br/>
</xsl:for-each>
Conclusion
I have
demonstrated with simple examples how selecting distinct nodes and grouping can
happen. You should be able to utilize this in your own stylesheets easily.
Related resources
Creating a Web.config Editor - Part 1 by Jason Gaylord
Creating a Web.config Editor - Part 2 by Jason Gaylord
Follow-up article: More XSLT & XPath grouping