Learning XML

6.4. Sorting

Elements often must be sorted to make them useful. Spreadsheets, catalogs, and surveys are a few examples of documents that require sorting. Imagine a telephone book sorted by three keys: last name, first name, and town. The document looks like this:

<telephone-book>
  ...
  <entry id="44456">
    <surname>Mentary</surname>
    <firstname>Rudy</firstname>
    <town>Simpleton</town>
    <street>123 Bushwack Ln</street>
    <phone>555-1234</phone>
  </entry>

  <entry id="44457">
    <surname>Chains</surname>
    <firstname>Allison</firstname>
    <town>Simpleton</town>
    <street>999 Leafy Rd</street>
    <phone>555-4321</phone>
  </entry>
  ...
</telephone-book>

By default, the transformation processes each node in the order it appears in the document. So the entry with id="44456" is output before id="44457". Obviously, that would not be in alphabetical order, so we need to sort the results somehow. It just so happens that we can do this with an element called <xsl:sort>. Here's how the document element's rule might look:

<xsl:template match="telephone-book">
  <xsl:apply-templates>
    <xsl:sort select="town"/>
    <xsl:sort select="surname"/>
    <xsl:sort select="firstname"/>
  </xsl:apply-templates>
</xsl:template>

There are three sorting axes here. First, all the results are sorted by town. Next, the entries are sorted by surname. Finally, the entries are sorted by first name.