XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (372 page)

Another situation where these rules matter is if one branch contains a call on an external function that has side effects. Writing such functions is something that's been left very much implementation-defined, but many XPath implementations will allow calls to external routines, and once they allow that, it's impossible to prevent such functions having arbitrary side effects. If you want to prevent a subexpression being evaluated because it has side effects, the only reliable way to ensure this is with an
if
expression; don't rely on
and
and
or
.

Note that there are no null values in XPath, as there are for example in SQL, and there is therefore no need for three-valued logic to handle unknown or absent data. Instead, you may need to test explicitly for absent values, as shown in some of the examples below.

Examples

Expression	Description
$x>3 and $x<8	Returns true if the value of variable $x is greater than 3 and less than 8.
@name and @address	Returns true if the context node has both a name and an address attribute. (Both the operands are sequences of nodes, which are converted to the xs:boolean true if they contain at least one node, and to false if they are empty).
string(@name) and string(@address)	Returns true if the context node has both a name and an address attribute and if neither is a zero-length string. (Both the operands are strings, which are converted to the xs:boolean true if their length is non-zero. If an attribute is absent, the sequence will be empty, and its string value will therefore be the empty string).
true()	A trivial AndExpr consisting of a single function call.
$x=5 or $x=10	Returns true if the variable $x has the value 5 or 10. This could also be written as $x = (5, 10) .
@name or @id	Returns true if the context node has a name attribute, an id attribute, or both.
not(@id) or @id=“”	Returns true if the context node has no id attribute or if it has an id attribute and the value is an empty string.
//para[position()=1 or position()=last()]	Selects the elements that are either the first or the last (or the only) children of their parent node.

Summary

This chapter described the following groups of XPath operators:

• Arithmetic operators,
  +
  ,
  -
  ,
  *
  ,
  div
  , and
  mod
  
• Value comparison operators
  eq
  ,
  ne
  ,
  lt
  ,
  le
  ,
  gt
  ,
  ge