Grouping Nodes (XSLT)

<?xml version="1.0"?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:output method="text" indent="no"/> <xsl:variable name="newline"> <xsl:text> </xsl:text> </xsl:variable> <xsl:template match="/"> <xsl:text>Addresses sorted by Zip Code</xsl:text> <xsl:value-of select="$newline"/> <xsl:for-each select="addressbook/address"> <xsl:sort select="zip"/> <xsl:if test="zip!=preceding-sibling::address[1]/zip"> <xsl:value-of select="$newline"/> <xsl:text>Zip code </xsl:text> <xsl:value-of select="zip"/> <xsl:text> (</xsl:text> <xsl:value-of select="city"/> <xsl:text>, </xsl:text> <xsl:value-of select="state"/> <xsl:text>): </xsl:text> <xsl:value-of select="$newline"/> </xsl:if> <xsl:if test="name/title"> <xsl:value-of select="name/title"/> <xsl:text> </xsl:text> </xsl:if> <xsl:value-of select="name/first-name"/> <xsl:text> </xsl:text> <xsl:value-of select="name/last-name"/> <xsl:value-of select="$newline"/> <xsl:value-of select="street"/> <xsl:value-of select="$newline"/> <xsl:value-of select="$newline"/> </xsl:for-each> </xsl:template> </xsl:stylesheet>

Addresses sorted by Zip Code Zip code 00218 (Winter Harbor, ME): Ms. Natalie Attired 707 Breitling Way Zip code 02718 (Skunk Haven, MA): Mary Backstayge 283 First Avenue Harry Backstayge 283 First Avenue Zip code 02930 (Lynn, MA): Ms. Amanda Reckonwith 930-A Chestnut Street Zip code 27318 (Boylston, VA): Mary McGoon 103 Bryant Street Mr. Chester Hasbrouck Frisby 1234 Main Street

<?xml version="1.0"?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:output method="xml" indent="no"/> <xsl:strip-space elements="*"/> <xsl:template match="/"> <addressbook> <xsl:for-each select="addressbook/address"> <xsl:sort select="name/last-name"/> <xsl:sort select="name/first-name"/> <xsl:copy-of select="."/> </xsl:for-each> </addressbook> </xsl:template> </xsl:stylesheet>

<?xml version="1.0"?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:output method="text" indent="no"/> <xsl:variable name="newline"> <xsl:text> </xsl:text> </xsl:variable> <xsl:template match="/"> <xsl:text>Addresses sorted by Zip Code</xsl:text> <xsl:value-of select="$newline"/> <xsl:for-each select="addressbook/address"> <xsl:sort select="zip"/> <xsl:sort select="name/last-name"/> <xsl:sort select="name/first-name"/> <xsl:variable name="lastZip" select="zip"/> <xsl:if test="not(preceding-sibling::address[zip=$lastZip])"> <xsl:text>Zip code </xsl:text> <xsl:value-of select="zip"/> <xsl:text>: </xsl:text> <xsl:value-of select="$newline"/> <xsl:for-each select="/addressbook/address[zip=$lastZip]"> <xsl:sort select="name/last-name"/> <xsl:sort select="name/first-name"/> <xsl:if test="name/title"> <xsl:value-of select="name/title"/> <xsl:text> </xsl:text> </xsl:if> <xsl:value-of select="name/first-name"/> <xsl:text> </xsl:text> <xsl:value-of select="name/last-name"/> <xsl:value-of select="$newline"/> <xsl:value-of select="street"/> <xsl:value-of select="$newline"/> <xsl:value-of select="$newline"/> </xsl:for-each> </xsl:if> </xsl:for-each> </xsl:template> </xsl:stylesheet>

Addresses sorted by Zip Code Zip code 00218: Ms. Natalie Attired 707 Breitling Way Zip code 02718: Harry Backstayge 283 First Avenue Mary Backstayge 283 First Avenue Zip code 02930: Ms. Amanda Reckonwith 930-A Chestnut Street Zip code 27318: Mary McGoon 103 Bryant Street Zip code 48392: Mr. Chester Hasbrouck Frisby 1234 Main Street

6.2.4. The <xsl:key> Approach

In this section, we'll look at using <xsl:key> to group items in an XML document. This approach is commonly referred to as the "Muench method," after Oracle XML Evangelist (and O'Reilly author) Steve Muench, who first suggested this technique. The Muench method has three steps:

Define a key for the property we want to use for grouping.
Select all of the nodes we want to group. We'll do some tricks with the key() and generate-id() functions to find the unique grouping values.
For each unique grouping value, use the key() function to retrieve all nodes that match it. Because the key() function returns a node-set, we can do further sorts on the set of nodes that match any given grouping value.

Well, that's how the technique works -- let's start building the stylesheet that makes the magic happen. The first step, creating a key function, is easy. Here's how it looks:

<xsl:key name="zipcodes" match="address" use="zip"/>

This <xsl:key> element defines a new index called zipcodes. It indexes <address> elements based on the value of the <zip> element they contain.

Now that we've defined our key, we're ready for the complicated part. We use the key() and generate-id() functions together. Here's the syntax, which we'll discuss extensively in a minute:

<xsl:for-each select="//address[generate-id(.)=
  generate-id(key('zipcodes', zip)[1])]">

Okay, let's take a deep, cleansing breath and start digging through this syntax. What we're selecting here is all <address> elements in which the automatically generated id matches the automatically generated id of the first node returned by the key() function when we ask for all <address> elements that match the current <zip> element.

Well, that's clear as crystal, isn't it? Let me try to explain that again from a slightly different perspective.

For each <address>, we use the key() function to retrieve all <address>es that have the same <zip>. We then take the first node from that node-set. Finally, we use the generate-id() function to generate an id for both nodes. If the two generated ids are identical, then the two nodes are the same.

Whew. Let me catch my breath.

If this <address> matches the first node returned by the key() function, then we know we've found the first <address> that matches this grouping value. Selecting all of the first values (remember, our previous predicate ends with [1]) gives us a node-set of some number of <address> elements, each of which contains one of the unique grouping values we need.

Well, that's how this technique works. At this point, we've got a way to generate a node-set that contains all of the unique grouping values; now we need to process those nodes. From this point, we'll do several things, all of which are comparatively simple:

Sort all nodes based on the grouping property. In this example, the property is the <zip> element. We start by selecting the first occurrence of every unique <zip> element in the document, then we sort those <zip> elements. Here's how it looks in the stylesheet:
```
<xsl:for-each 
  select="//address[generate-id(.)=generate-id(key('zipcodes', zip)[1])]">
  <xsl:sort select="zip"/>
```
The outer <xsl:for-each> element selects all the unique values of the <zip> element. Next, we use the key() function to retrieve all <address> elements that match the current <zip> element:
```
<xsl:for-each select="key('zipcodes', zip)">
```

The key() function gives us a node-set of all matching <address> elements. We sort that node-set based on the <last-name> and <first-name> elements, then process them in turn:

    <xsl:sort select="name/last-name"/>
    <xsl:sort select="name/first-name"/>
    <tr>
      <xsl:if test="position() = 1">
        <td valign="center" bgcolor="#999999">
          <xsl:attribute name="rowspan">
            <xsl:value-of select="count(key('zipcodes', zip))"/>
          </xsl:attribute>
          <b>
            <xsl:text>Zip code </xsl:text><xsl:value-of select="zip"/>
          </b>
        </td>
      </xsl:if>
      <td align="right">
        <xsl:value-of select="name/first-name"/>
        <xsl:text> </xsl:text>
        <b><xsl:value-of select="name/last-name"/></b>
      </td>
      <td>
        <xsl:value-of select="street"/>
        <xsl:text>, </xsl:text>
        <xsl:value-of select="city"/>
        <xsl:text>, </xsl:text>
        <xsl:value-of select="state"/>
        <xsl:text> </xsl:text>
        <xsl:value-of select="zip"/>
      </td>
    </tr>
  </xsl:for-each>
</xsl:for-each>

We generate a table cell that contains the Zip Code common to all addresses, creating a rowspan attribute based on the number of matches for the current Zip Code. From there, we write the other data items into table cells.

Here's our complete stylesheet:

<?xml version="1.0"?>

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:output method="html" indent="no"/>

  <xsl:key name="zipcodes" match="address" use="zip"/>

  <xsl:template match="/">
    <table border="1">
      <xsl:for-each select="//address[generate-id(.)=
        generate-id(key('zipcodes', zip)[1])]">
        <xsl:sort select="zip"/> 
        <xsl:for-each select="key('zipcodes', zip)">
          <xsl:sort select="name/last-name"/>
          <xsl:sort select="name/first-name"/>
          <tr>
            <xsl:if test="position() = 1">
              <td valign="center" bgcolor="#999999">
                <xsl:attribute name="rowspan">
                  <xsl:value-of select="count(key('zipcodes', zip))"/>
                </xsl:attribute>
                <b>
                  <xsl:text>Zip code </xsl:text><xsl:value-of select="zip"/>
                </b>
              </td>
            </xsl:if>
            <td align="right">
              <xsl:value-of select="name/first-name"/>
              <xsl:text> </xsl:text>
              <b><xsl:value-of select="name/last-name"/></b>
            </td>
            <td>
              <xsl:value-of select="street"/>
              <xsl:text>, </xsl:text>
              <xsl:value-of select="city"/>
              <xsl:text>, </xsl:text>
              <xsl:value-of select="state"/>
              <xsl:text> </xsl:text>
              <xsl:value-of select="zip"/>
            </td>
          </tr>
        </xsl:for-each>
      </xsl:for-each>
    </table>
  </xsl:template>

</xsl:stylesheet>

When we view the generated HTML document in a browser, it looks like Figure 6-1.

Figure 6-1. HTML document with grouped items

Notice how the two <xsl:for-each> and the various <xsl:sort> elements work together. The outer <xsl:for-each> element selects the unique values of the <zip> element and sorts them; the inner <xsl:for-each> element selects all <address> elements that match the current <zip> element, and then sorts them by <last-name> and <first-name>.

6.2. Grouping Nodes

6.2.1. Our First Attempt

6.2.2. A Brute-Force Approach

6.2.3. Grouping with <xsl:variable>

6.2.4. The <xsl:key> Approach

Figure 6-1. HTML document with grouped items