XML namespaces and qualified names in DB2 XPath
DB2® XPath uses XML namespaces to prevent naming collisions. An XML namespace is a collection of names that is identified by a namespace URI. Namespaces provide a way of qualifying names that are used for elements, attributes, data types, and functions in XPath.
Names in XPath are called QNames (qualified names) and conform to the syntax that is defined in the W3C Recommendation Namespaces in XML. A QName consists of an optional namespace prefix and a local name. The namespace prefix, if present, is bound to a URI and provides a shortened form of the URI. During query processing, DB2 XPath expands the QName by resolving the URI that is bound to the namespace prefix. The expanded QName includes the namespace URI and a local name. Two QNames are equal if they have the same namespace URI and local name. This means that two QNames can match even if they have different prefixes provided that the prefixes are bound to the same namespace URI.
pfx1
is
a prefix that is bound to some URI. pfx2
is a prefix
that is bound to a different URI. c
is the local
name for all three elements:<a xmlns:pfx1="uri1" xmlns:pfx2="uri2">
<b>
<pfx1:c>C</pfx1:c>
<pfx2:c>B</pfx2:c>
<c>A</c>
</b>
</a>
The elements in this example share the same
local name, c
, but naming conflicts do not occur
because the elements exist in different namespaces. During expression
processing, the name pfx1:c
is expanded into a name
that includes the URI bound to pfx1
(uri1
)
and the local name, c
. Likewise, the name pfx2:c
is
expanded into a name that includes the URI bound to pfx2
(uri2
)
and the local name, c
. The element c
,
which has an empty prefix, is bound to the default element namespace
because no prefix is specified. An error is raised if a name uses
a prefix that is not bound to a URI.The namespace prefix must be an NCName (non-colonized name). An XML NCName is similar to an XML Name except that NCName cannot include a colon.
Some namespaces are predeclared; others can be added through declarations in the XPath expression prolog. DB2 XPath includes the following predeclared namespace prefixes:
Prefix | URI | Description |
---|---|---|
xs | http://www.w3.org/2001/XMLSchema | XML Schema namespace |
xsi | http://www.w3.org/2001/XMLSchema-instance | XML Schema instance namespace |
fn | http://www.w3.org/2005/xpath-functions | Default function namespace |
xdt | http://www.w3.org/2005/xpath-datatypes | XPath type namespace |
db2-fn | http://www.ibm.com/xmlns/prod/db2/functions | DB2 function namespace |
- The following namespace information is available in the static
context:
- In-scope namespaces are a set of prefix and URI pairs.
The in-scope namespaces are used for resolving prefixes that are used
in QNames in an XPath expression. In-scope namespaces come from the
following sources:
- Namespace declarations in an XPath expression
- The XMLNAMESPACES DB2 built-in function in the XMLELEMENT, XMLFOREST, or XMLTABLE DB2 built-in function
- Default element or type namespace is the namespace
that is used for any unprefixed QName that appears where an element
or type name is expected. The initial default element or type namespace
is the default namespace that is provided by a
declare default element namespace
clause in the prolog of an XPath expression. - Default function namespace is the namespace that is associated with built-in functions: http://www.w3.org/2003/11/xpath-functions. There are no user-defined functions in XPath.
- In-scope namespaces are a set of prefix and URI pairs.
The in-scope namespaces are used for resolving prefixes that are used
in QNames in an XPath expression. In-scope namespaces come from the
following sources: