XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (519 page)

For both the
id()
and
key()
functions, the required value of the ID or key can be specified as either a literal or a variable reference (in the form
$
QName
). For the
id()
function, the only kind of literal that makes sense is a string literal (for example,
“E-102”
). For the key function, numeric literals also make sense if the key values are numeric. It is not possible to supply literals for other data types such as
xs:date
values.

With the
key()
function, the first argument is the name of the key. This must be specified as a string literal, and it must match the name of a key defined in the stylesheet.

Usage

This facility provides an equivalent to the ability in Cascading Style Sheets (CSS) to define a style for a specific node in the source document. Here are some ways it can be used:

• If for a particular source document you want to use a general-purpose stylesheet, but want to override its behavior for certain selected nodes, you can write a stylesheet that imports the general-purpose one and then write the overriding rules in the form of templates that match specific identified elements in the source document.
  
• Sometimes the source document is generated dynamically from a database. Perhaps there is something in the source document you want to highlight, say the search term that was used to locate this record. You could flag this item while generating the source document by giving it a special ID attribute value known to the stylesheet.
  

In practice, this construct isn't as useful as it might seem. Even though XSLT 2.0 has made it a lot more flexible by allowing the value to be specified as a variable (which in general is likely to be a stylesheet parameter), this form of pattern still achieves nothing that can't be achieved just as easily with a predicate, and it is unlikely to be any more efficient.

For example, if

elements are keyed on their ISBN property, which is implemented as a child element, then the following declarations are equivalent:

Of course, there may be a performance difference between the two, but this depends on how the XSLT processor is implemented. There is certainly no intrinsic reason why the predicate should be less efficient.

Examples

id(‘figure1’)	Matches a node with an ID attribute equal to the string ‘figure1’ . An attribute is an ID attribute if it is defined in the Document Type Definition (DTD) or schema as having type ID (the name of the attribute is irrelevant).
key(‘empnr’,$ pers)	Matches a node having a value of $pers for the key named empnr , where $pers is typically a stylesheet parameter.

The following example shows how this feature can be used in a stylesheet.