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.

XQuery

A query that invokes XQuery directly begins with the keyword XQUERY. This keyword indicates that XQuery is being used and that the DB2 server must therefore use case sensitivity rules that apply to the XQuery language. Error handling is based on the interfaces that are used to process XQuery expressions. XQuery errors are reported with an SQLCODE and SQLSTATE in the same way that SQL error errors are reported. No warnings are returned from processing XQuery expressions. XQuery obtains data by calling functions that extract XML data from DB2 tables and views. XQuery can also be invoked from an SQL query. In this case, the SQL query can pass XML data to XQuery in the form of bound variables. XQuery supports various expressions for processing XML data and for constructing new XML objects such as elements and attributes. The programming interface to XQuery provides facilities similar to those of SQL to prepare queries and retrieve query results.

SQL

SQL provides capabilities to define and instantiate values of the XML data type. Strings that contain well-formed XML documents can be parsed into XML values, optionally validated against an XML schema, and inserted or updated in tables. Alternatively, XML values can be constructed by using SQL constructor functions, which convert other relational data into XML values. Functions are also provided to query XML data by using XQuery and to convert XML data into a relational table for use within an SQL query. Data can be cast between SQL and XML data types in addition to serializing XML values into string data.

SQL/XML provides the following functions and predicates for calling XQuery from SQL:

XMLQUERY

XMLQUERY is a scalar function that takes an XQuery expression as an argument and returns an XML sequence. The function includes optional parameters that can be used to pass SQL values to the XQuery expression as XQuery variables. The XML values that are returned by XMLQUERY can be further processed within the context of the SQL query.

XMLTABLE

XMLTABLE is a table function that uses XQuery expressions to generate an SQL table from XML data, which can be further processed by SQL.

XMLEXISTS

XMLEXISTS is an SQL predicate that determines if an XQuery expression returns a sequence of one or more items (and not an empty sequence).