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SQL/XML, XQuery, and Native XML Programming Languages
Most web applications have connections to databases and use XML to transfer data from the database to the web application and vice versa. Every major database vendor has proprietary extensions for using XML with relational databases, but they take completely different approaches, and there is no interoperability between them. Many developers need to be able to write applications that work for databases from multiple vendors.
XQuery and SQL/XML are two standards that use declarative, portable queries to return XML by querying data. In both standards, the XML can have any desired structure, and the queries can be arbitrarily complex. XQuery is XML-centric, while SQL/XML is SQL-centric.
SQL/XML is an extension of SQL that is part of ANSI/ISO SQL 2003. It lets SQL queries create XML structures with a few powerful XML publishing functions. For a SQL programmer, SQL/XML easy to learn because it involves only a few small additions to the existing SQL language. Since SQL is a mature language, there are a lot of tools and infrastructure for SQL. For instance, SQL/XML uses JDBC to return results, and there is currently no equivalent standard API for XQuery. SQL also has functionality not yet found in XQuery, such as updates or stored procedures.
Note: SQL/XML is completely different from Microsoft's SQLXML, a proprietary technology used in SQL Server. The similarity in names has caused a great deal of confusion in the industry.
XQuery is a completely new query language that uses XML as the basis for its data model and type system. It is being developed in the XML Query Working Group, which is a part of the World Wide Web Consortium. In this paper, we characterize XQuery as a "Native XML Programming Language". XQuery is based on XML in the same way that SQL is based on the relational model or object oriented languages are based on the object oriented model - XML is central to its type system, in which elements and attributes are just as fundamental as integers and strings. Although XQuery per se has no concept of relational data, several products and many projects provide ways to query relational data using an XML view of the database, and the need to make this possible has influenced the design of XQuery throughout its development. XQuery allows you to work in the XML world no matter what type of data you're working with - relational, XML or object data. XQuery is ideal for native XML programming. When used with XML views of relational data, it is also ideal for queries data that must represent results as XML, to query XML stored inside or outside the database, or to span relational and XML sources.
For queries based only on relational data, SQL/XML and XQuery have substantially similar functionality. However, the way in which a given task is done is quite different, since SQL/XML operates on the borderline between SQL and XML, and XQuery lives in a purely XML world. Even when the data is all relational, the two languages appeal to very different audiences - SQL/XML is very much an extension of SQL, designed for SQL programmers, and XQuery takes a purely XML view of the world. For queries that span relational and XML sources, XQuery has important advantages.
This talk uses a series of concrete queries written in each language to show the advantages of each. It explains why we need both languages, discussing the ways in which the languages differ and in which they overlap. It also explores the role of SQL/XML mappings as a way of creating XML views for XQuery
Table of Contents
XML and relational databases are tightly wed in most web applications, but a look at the two models shows that it is an unlikely marriage - though a necessary one.
XML applications that use relational data can choose from four approaches, each with distinct advantages and disadvantages.
SQL/XML refers to the XML extensions of SQL. These are developed by INCITS H2.3, with participation from Oracle, IBM, Microsoft (which does not plan to implement SQL/XML), Sybase, and DataDirect Technologies.
The XML Publishing Functions are the part that are directly used in a SQL query. Let's compare a traditional SQL query with one that uses an XML publishing function.
The XML Datatype is a datatype in the same way that integer, date, or CLOB are datatypes in SQL. Since SQL/XML allows a query to create XML instances, there must be a datatype that corresponds to these instances.
The XML publishing functions use SQL values to create XML values, and these XML values have W3C XML Schema types.
The XQuery language was designed for querying or processing XML. Just as a traditional SQL query takes a set of tables as input and returns an XML table as its result, XQuery takes sequences of XML nodes as input and evaluates to a sequence of XML nodes.
XQuery is a language designed for integrating data from multiple sources, including XML sources like documents or web messages and databases.
An XML document can be represented using objects, and this is precisely the approach taken by DOM and JDOM.
To many intelligent and articulate XML programmers, "XML is just Unicode with pointy brackets" is almost a statement of faith.
A Native XML Programming Language must provide the fundamental operations needed for XML. Some of these operations are required because of the structure of XML itself.
Some people seem to believe that the purpose of XQuery is largely the same as that of SQL/XML - to allow XML structures to be created from relational data.
XQuery, when combined with a SQL/XML view of a relational database, is extremely good for processing XML together with relational data.
SQL programmers are used to using APIs such as ODBC or JDBC to set up the environment, execute queries, and do processing in the business domain using the data returned by a query.
Although SQL/XML and XQuery are both XML query standards, they are based on quite different models, and fit best in different architectures.
Detailed list of sources for this white page can be found here in the bibliography section.
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