Set Operations
XML Path Language (XPath) supports the set operation |
.
Union (|) Operator
The |
, or union, operator returns the union of its two operands, which must be node-sets. For example, //author | //publisher
returns a node-set that combines all the //author
nodes and all the //publisher
nodes. Multiple union operators can be chained together to combine multiple node-sets. For example, //author | //publisher | //editor | //book-seller
returns a node-set containing all //author
, //publisher
, //editor
, and //book-seller elements
. The union operator preserves document order and does not return duplicates.
Examples
Expression | Refers to |
---|---|
first-name | last-name |
A node set containing <first-name> and <last-name> elements in the current context. |
(bookstore/book | bookstore/magazine) |
A node set containing <book> or <magazine> elements inside a <bookstore> element. |
book | book/author |
A node set containing all <book> elements and all <author> elements within <book> elements. |
(book | magazine)/price |
The node set containing all <price> elements of either <book> or <magazine> elements. |
Example
The following example illustrates the effect of the union operator.
XML File (test.xml)
<?xml version="1.0"?> <test> <x a="1"> <x a="2" b="B"> <x> <y>y31</y> <y>y32</y> </x> </x> </x> </test>
XSLT File (test.xsl)
The following XSLT style sheet selects all the <x>
elements whose a
attribute is equal to 2
, plus those <x>
elements that have no attributes.
<?xml version='1.0'?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:output method="xml" omit-xml-declaration="yes" indent="yes"/> <!-- Suppress text nodes not covered in subsequent template rule. --> <xsl:template match="text()"/> <!-- Handles a generic element node. --> <xsl:template match="*"> <xsl:element name="{name()}"> <xsl:apply-templates select="*|@*" /> <xsl:if test="text()"> <xsl:value-of select="."/> </xsl:if> </xsl:element> </xsl:template> <!-- Handles a generic attribute node. --> <xsl:template match="@*"> <xsl:attribute name="{name()}"> <xsl:value-of select="."/> </xsl:attribute> </xsl:template> <xsl:template match="/test"> <xsl:apply-templates select="//x[@a=2 | //x[not(@*)]/> </xsl:template> </xsl:stylesheet>
The transformation yields the following result:
<x a="2" b="B"> <x> <y>31</y> <y>y32</y> </x> </x> <x> <y>y31</y> <y>y32</y> </x>
Precedence
Precedence order (from highest precedence to lowest) between Boolean and comparison operators is shown in the following table.
1 | ( ) |
Grouping |
2 | [ ] |
Filters |
3 | /
|
Path operations |
4 | <
|
Comparisons |
5 | =
|
Comparisons |
6 | | |
Union |
7 | not() |
Boolean not |
8 | and |
Boolean and |
9 | or |
Boolean or |
Example
The following example illustrates the effect of the operator precedence listed above.
XML File (test.xml)
<?xml version="1.0"?> <test> <x a="1"> <x a="2" b="B"> <x> <y>y31</y> <y>y32</y> </x> </x> </x> <x a="1"> <x a="2"> <y>y21</y> <y>y22</y> </x> </x> <x a="1"> <y>y11</y> <y>y12</y> </x> <x> <y>y03</y> <y>y04</y> </x> </test>
Basic XSLT File (test.xsl)
We will use this basic XSLT file as a starting point for the series of illustrations that follow.
<?xml version='1.0'?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:output method="xml" omit-xml-declaration="yes" indent="yes"/> <!-- Suppress text nodes not covered in subsequent template rule. --> <xsl:template match="text()"/> <!-- Handles a generic element node. --> <xsl:template match="*"> <xsl:element name="{name()}"> <xsl:apply-templates select="*|@*" /> <xsl:if test="text()"> <xsl:value-of select="."/> </xsl:if> </xsl:element> </xsl:template> <!-- Handles a generic attribute node. --> <xsl:template match="@*"> <xsl:attribute name="{name()}"> <xsl:value-of select="."/> </xsl:attribute> </xsl:template> </xsl:stylesheet>
Case 0. Test run
You can add the following template-rule to the XSLT style sheet.
<xsl:template match="/test"> <xsl:apply-templates select="*|@*/> </xsl:template>
This will produce an XML document identical to the original one, without the <?xml version="1.0"?>
processing instruction.
The following cases show different ways of writing this template rule. The point is to show the order in which the XPath operators bind to an element.
Case 1: () binds tighter than []
The following template rule selects the first <y>
element in the document order, from all the <y>
elements in the source document.
<xsl:template match="/test"> <xsl:apply-templates select="(//y)[1]"/> </xsl:template>
The result is as follows:
<y>y31</y>
Case 2: [] binds tighter than / or //
The following template rule selects all the <y>
elements that are the first among their siblings.
<xsl:template match="/test"> <xsl:apply-templates select="//y[1]"/> </xsl:template>
The result is as follows:
<y>y31</y> <y>y21</y> <y>y11</y> <y>y03</y>
Case 3: and, not
The following template rule selects all the <x>
elements that have no <x>
child elements, that have an <x>
parent element, and that do not have any attributes.
<xsl:template match="/test"> <xsl:apply-templates select= "//x[./ancestor::*[name()='x'] and *[name()!='x'] and not(@*)]"/> </xsl:template>
The result is a single <x>
element, listed below with its children:
<x> <y>y31</y> <y>y32</y> </x>
Case 4: or, and, not
The following template rule selects each <x>
elements that is a child of an <x>
element; or, that is not a parent of an <x>
element and has no attributes.
<xsl:template match="/test"> <xsl:apply-templates select= "//x[./ancestor::*[name()='x'] or *[name()!='x'] and not(@*)]"/> </xsl:template>
The result is a node set containing the following <x>
elements, listed below with its children:
<x a="2" b="B"> <x> <y>y31</y> <y>y32</y> </x> </x> <x> <y>y31</y> <y>y32</y> </x> <x a="2"> <y>y21</y> <y>y22</y> </x> <x> <y>y03</y> <y>y04</y> </x>
Case 5: and, or, not
The following template rule selects each <x>
element that is a child of an <x>
element but not a parent of an <x>
element; or, that has no attributes.
<xsl:template match="/test"> <xsl:apply-templates select= "//x[./ancestor::*[name()='x'] and *[name()!='x'] or not(@*)]"/> </xsl:template>
The result is a node set containing the following <x>
elements, listed below with its children:
<x> <y>y31</y> <y>y32</y> </x> <x a="2"> <y>y21</y> <y>y22</y> </x> <x> <y>y03</y> <y>y04</y> </x>