Grouping in XSLT 3

Grouping in XSLT 3, the current and latest version of XSLT since 2017, can be achieved primarily using the xsl:for-each-group instruction where you select the items to be grouped, the grouping population, with the select attribute and then have three different ways to group the population:

1. the group-by attribute selecting a sequence of grouping keys by which the grouping population is to be grouped
2. the group-adjacent attribute selecting a sequence of grouping keys by which adjacent items in the population are to be grouped
3. the group-starting-with or group-ending-with attributes defining patterns to identify and start groups based on pattern matching of the initial (group-starting-with) or final (group-ending-with) member of a group

Access to the items of a currently processed group is given by the current-group() function, access to the current grouping key by the current-grouping-key() function.

The XSLT 3 specification gives at least one example for each grouping approach directly in the specification section https://www.w3.org/TR/xslt-30/#grouping-examples; below you find links to XSLT fiddles based on these examples:

• grouping items based on a single common value, using group-by: https://xsltfiddle.liberty-development.net/ncdD7mq/1
• grouping items based on a sequence of values as the grouping key, i.e. a composite key, using group-by: https://xsltfiddle.liberty-development.net/pPzifps
• grouping alternate sequences of items using group-adjacent: https://xsltfiddle.liberty-development.net/3NJ38Zc
• identifying a group by its initial element, using group-starting-with: https://xsltfiddle.liberty-development.net/bnnZWa
• identifying a group by its final element, using group-ending-with: https://xsltfiddle.liberty-development.net/6r5Gh3e
• grouping items into several groups using group-by if the group-by expression gives a sequence of more than one value: https://xsltfiddle.liberty-development.net/94rmq6g

Of course the different approaches can, for more complex tasks, be combined by nesting xsl:for-each-group instructions.

XSLT 3 with XPath 3.1 support can also group JSON represented as XPath 3.1 maps and arrays, here is a list of showing the previous XML grouping samples now using JSON input and directly grouping the map/array data structure returned by the parse-json function:

• grouping items based on a single common value, using group-by: https://xsltfiddle.liberty-development.net/jyRYYip
• grouping items based on a sequence of values as the grouping key, i.e. a composite key, using group-by: https://xsltfiddle.liberty-development.net/ncdD7mr
• identifying a group by its initial item, using group-starting-with: https://xsltfiddle.liberty-development.net/pPzifpt
• identifying a group by its final item, using group-ending-with: https://xsltfiddle.liberty-development.net/bnnZWb
• grouping items into several groups using group-by if the group-by expression gives a sequence of more than one value: https://xsltfiddle.liberty-development.net/3NJ38Zd
• grouping items into several groups using group-by if the group-by expression gives a sequence of more than one value, this time with JSON output: https://xsltfiddle.liberty-development.net/3NJ38Zd/1

Positional grouping

Positional grouping can be used to split a sequence of items into groups of a certain size; in XSLT 2 or 3 this is easily achieved using e.g. for-each-group select="foo" group-adjacent="(position() - 1) idiv $chunk-size": https://xsltfiddle.liberty-development.net/asoTK9

https://martin-honnen.github.io/xslt/generic-positional-grouping-functions.xsl is a compact XSLT 3 module using higher-order functions importable by other code that needs to split up a sequence of items using positional grouping, you could use it as follows (Saxon-JS 2, Saxon 10 or higher all editions, Saxon 9.8 or higher PE and EE, Altova XML 2017 R3 or later)

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    version="3.0"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:mf="http://example.com/mf"
    xmlns:ex="http://example.com/ex"
    exclude-result-prefixes="#all"
    expand-text="yes">

  <xsl:import href="https://martin-honnen.github.io/xslt/generic-positional-grouping-functions.xsl"/>

  <xsl:function name="ex:wrap-rows" as="element()">
    <xsl:param name="group" as="item()*"/>
    <xsl:param name="pos" as="xs:integer"/>
    <xsl:param name="wrapper-name" as="xs:QName"/>
    <xsl:element name="{$wrapper-name}" namespace="{namespace-uri-from-QName($wrapper-name)}">
      <xsl:attribute name="index" select="$pos"/>
      <xsl:sequence select="$group"/>
    </xsl:element>
  </xsl:function>

  <xsl:output method="xml" indent="yes"/>

  <xsl:template match="root">
    <root>
      <xsl:sequence select="mf:group-chunks(item, 3, ex:wrap-rows(?, ?, QName('', 'chunk')))"/>
    </root>
  </xsl:template>

  <xsl:template match="/">
    <xsl:next-match/>
    <xsl:comment>Run with {system-property('xsl:product-name')} {system-property('xsl:product-version')} {system-property('Q{http://saxon.sf.net/}platform')}</xsl:comment>
  </xsl:template>

</xsl:stylesheet>

to e.g. wrap any chunk into a container element.

Grouping in XSLT 2

Grouping in XSLT 2 also works with the xsl:for-each-group instruction as in XSLT 3, the main restrictions are that XSLT 2 does not support composite grouping keys (so to group on various items you need to concat or string-join them as single value (e.g. composite="yes" group-by="foo, bar" in XSLT 3 needs to be done as group-by="string-join((foo, bar), '|')")) and that pattern matching can only be done on nodes, not on primitive values.

The XSLT 2 specification gives at least one example for each grouping approach directly in the specification section https://www.w3.org/TR/xslt20/#grouping-examples; below you find links to XSLT fiddles based on these examples:

• grouping items based on a single common value, using group-by: https://xsltfiddle.liberty-development.net/gW3LG7m
• grouping items based on a composite, concatenated grouping key using group-by: https://xsltfiddle.liberty-development.net/ncdD7ms
• grouping adjacent nodes using group-adjacent: https://xsltfiddle.liberty-development.net/3NJ38Ze
• grouping nodes based on the pattern of the initial node in a group, using group-starting-with: https://xsltfiddle.liberty-development.net/pPzifpu
• grouping nodes based on the pattern of the final node in a group using group-ending-with: https://xsltfiddle.liberty-development.net/bnnZWc
• group-by example adding an item to several groups if grouping key expression returns a sequence of more than one value: https://xsltfiddle.liberty-development.net/6r5Gh3g

Grouping in XSLT 1.0

The preferred solution in XSLT 1.0 is to use the Muenchian grouping method:
http://www.jenitennison.com/xslt/grouping/muenchian.html