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XSLT, 2nd Edition by Doug Tidwell

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The Details on the <xsl:sort> Element

Now that we’ve seen a couple of examples of how <xsl:sort> works, we’ll go over its syntax, its attributes, and where you can use it.

What’s the deal with that syntax?

I’m so glad you asked that question. One thing the XSLT working group could have done is something like this:

<xsl:for-each select="addressbook/address" sort-key-1="name/last-name" 
  sort-key-2="name/first-name"/>

The problem with this approach is that no matter how many sort-key-x attributes you define, out of sheer perverseness, someone will cry out that they really need the sort-key-8293 attribute. To avoid this messy issue, the XSLT designers decided to let you specify the sort keys by using a number of <xsl:sort> elements. The first is the primary sort key, the second is the secondary sort key, the 8293rd one is the eight-thousand-two-hundred-and-ninety-third sort key, etc.

Well, that’s why the syntax looks the way it does, but how does it actually work? When I first saw this syntax:

<xsl:for-each select="addressbook/address">
  <xsl:sort select="name/last-name"/>
  <xsl:sort select="name/first-name"/>
  ...
</xsl:for-each>

I thought it meant that all the nodes were sorted during each iteration through the <xsl:for-each> element. That seemed incredibly inefficient; if you’ve sorted all the nodes, why re-sort them each time through the <xsl:for-each> element? Actually, the XSLT processor handles all <xsl:sort> elements before it does anything, then it processes the <xsl:for-each> element ...

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