url - Universität zu Lübeck

2.4. XUPDATE: XML MODIFICATION LANGUAGE 35
2.4 XUpdate: XML Modification Language
With XPath and XQuery we are able to select nodes and create an XML output
with content from an XML document. Modifications in an existing document can
be performed neither with XPath nor XQuery. With XSLT [129] we can transform
one document A into a document B and perform some changes to B. But because
XSLT works like a stylesheet the modifications do not take place in the original
document. Furthermore, XSLT is Turing-complete and can be regarded as a cryptic
but full programming language that does not allow comfortable ad-hoc queries
like SQL for instance.
In this thesis XUpdate [134] is presented as a proposal for an XML modification
language comparable to SQL. Although XUpdate relies strongly on XPath it
was not designed by the W3C. The latest XUpdate Working Draft was published
in 2000 by the XML:DB Initiative. No enhancement or updates of XUpdate were
presented recently. Nonetheless, XUpdate is used by some XML database management
systems like XIndice [8]. A successor project of XUpdate is Lexus [6], an
update language also using XML syntax and XPath for navigation and selection.
In this thesis we give a brief and informal introduction to XUpdate - as one representative
of an ad-hoc XML modification language. Basically, there are six different
types of modifications that are presented in the following by using examples.
For the reason of simplicity, elements and attributes are summarized to nodes.
Constructing and adding new nodes
XUpdate allows to add one or multiple new nodes to an existing document. The
insert position can be before or after a designated node or the new node(s) may be
appended to a list of existing nodes. This is shown in the next two examples.
1
2
3 Harddisks
4
5
Figure 2.13: XUpdate append operation
As one can see XUpdate expressions are well-formed XML documents themselves.
The root-node is an element called modifications that belongs to the namespace
⁀xupdate. The second line selects the /site/categories element and adds one
new category element with the value Harddisks. In this command an implicit
element creation takes place.
The second example inserts a full XML fragment consisting of one item-node with
an attribute and a complex content.