DB2 Version 10.1 for Linux, UNIX, and Windows

Querying XML data

You can query or retrieve XML data stored in the database through two main query languages, either by using each language on its own or by using a combination of the two.

The following options are available to you:

XQuery expressions only
XQuery expressions that invoke SQL statements
SQL statements only
SQL statements that executes XQuery expressions

These various methods allow you to query or retrieve XML and other relational data from either an SQL or XQuery context.

Pieces of or entire XML documents can be queried and retrieved using these methods. Queries can return fragments or entire XML documents, and results returned from queries can be limited by using predicates. Because queries on XML data return XML sequences, a query's result can be used in the construction of XML data as well.

Introduction to XQuery
XQuery is a functional programming language that was designed by the World Wide Web Consortium (W3C) to meet specific requirements for querying and modifying XML data.
Retrieving DB2 data with XQuery functions
In XQuery, a query can call one of the following functions to obtain input XML data from a DB2® database: db2-fn:sqlquery and db2-fn:xmlcolumn.
Introduction to querying XML data with SQL
XML data can be queried using an SQL fullselect or with the SQL/XML query functions of XMLQUERY and XMLTABLE. The XMLEXISTS predicate can also be used in SQL queries on XML data.
Comparison of XQuery to SQL
DB2 databases support storing well-formed XML data in a column of a table and retrieving the XML data from the database by using SQL, XQuery, or a combination of SQL and XQuery. Both languages are supported as primary query languages, and both languages provide functions for invoking the other language.
Comparison of methods for querying XML data
Because XML data can be queried in a number of ways, using XQuery, SQL, or a combination of these, the method to choose can differ depending on your situation. The following sections describe conditions that are advantageous for a particular query method.
Specifying XML namespaces
In an XML document, an XML namespace is optional and is used as a prefix for node names in the XML document. To access the nodes in an XML document that uses a namespace, your XQuery expressions must also specify the same namespace as part of the node name.
Example: Changing the namespace prefix of an element
These examples demonstrate how add and remove the namespace binding assigned to the QName of an element.
XMLQUERY function overview
Use the XMLQUERY SQL scalar function to execute an XQuery expression from within an SQL context. You can pass variables to the XQuery expression specified in XMLQUERY. XMLQUERY returns an XML value, which is an XML sequence. This sequence can be empty or contain one or more items.
XMLTABLE function overview
The XMLTABLE SQL table function returns a table from the evaluation of XQuery expressions. XQuery expressions normally return values as a sequence, however, use the XMLTABLE function to execute XQuery expressions and return values as a table instead.
XMLEXISTS predicate when querying XML data
The XMLEXISTS predicate determines whether an XQuery expression returns a sequence of one or more items. If the XQuery expression specified in this predicate returns an empty sequence, XMLEXISTS returns false; otherwise, true is returned.
Passing parameters between SQL statements and XQuery expressions
When issuing combined SQL statements and XQuery expressions, you can pass data between the statements and expressions to modify the execution of the statements and expressions.
Data retrieval with XQuery
XQuery expressions can be executed either using XQuery as the primary language or using SQL with the XMLQUERY SQL function as the primary language. When an XQuery expression is executed using either method, an XML sequence is returned.
Guidelines for matching indexes with queries overview
This section provides you with some guidelines and examples for matching queries with indexes over XML data.
Full-text search in XML documents
Full-text search of natively stored XML data is available through DB2 Net Search Extender.
Retrieval of data in XML columns to earlier DB2 clients
If you retrieve data from an XML column to a client that is at an earlier release than DB2 Version 9.1, your database client cannot handle XML data.
SQL/XML publishing functions for constructing XML values
You can construct XML values, which do not necessarily have to be well-formed XML documents, by combining those publishing functions that correspond to the components you want in the resulting the XML value. The functions must be specified in the order that you want the results to appear.
Transforming with XSLT stylesheets
The standard way to transform XML data into other formats is by Extensible Stylesheet Language Transformations (XSLT). You can use the built-in XSLTRANSFORM function to convert XML documents into HTML, plain text, or different XML schemas.
Differences in an XML document after storage and retrieval
After storing an XML document in a DB2 database, the retrieved document might not be exactly the same as the original document. This behavior is defined by the XML and SQL/XML standard and matches that of the Xerces open source XML parser.

Related concepts:

XML input and output overview