Download Presentation summary: XML views in relational databases

Presentation summary: XML views in relational databases
Done by: El Hazoui Ilias
Supervised by: Dr. Haddouti
7 October 2003
1. Definition of XML view:
The definition of XML view is somewhat the same as the concept of view in the
relational databases. However, the result of the XML view is an XML document. The
XML view describes the shape of an XML document, even though the XML does not
really exist because it is created on the fly from the relational engine. In addition, we
can query this XML view as though it were a real XML document; the query,
together with the view, determines the shape of the resulting XML.
2. The importance of XML view:
XML has emerged as the standard data exchange format for internet-based business
applications. This has created the need to publish existing business data, stored in
relational databases, as XML. A general way to publish relational data as XML is to
provide XML views over relational data, and allow business partners to query these
views using an XML query language.
3. XML view architecture:
A possible architecture is based on three components:
 The data server that may be a database, an XML repository, or any source capable
of exporting XML data.
 The View server that restructures data to construct the view, possibly deals with
access rights, and integrates data from several sources.
 An XML view document that is handled by a standard Web browser and interacts
with view server to obtain data.
XML View Architecture
Vie w
Specification
Data Server
Vie w
Server
XML
Repository
Vie w Docume nts
(possibly virtual)
XML
Stylesheet
Web
Browser
Vie w
Pages
External
Application
4. The implementation of XML views:
Many universities and research laboratories are till working on this topic to find a
universal standard to make relational data viewable as XML documents; here I
present some projects that support XML views:
 The Mediation of Information using XML (MIX) project, which is a joint effort
between the San Diego Supercomputer Center and the Database Lab at the
University of California San Diego. They are developing a wrapper-mediator
system which employs XML as a means for information modeling, as well as
interchange, across heterogeneous information sources including GIS systems,
and web sites with HTML pages. However, this project is not optimized for
RDBMS’s.
 Most commercial database systems provide a way to create views of relational
data. However, most of these systems do not support queries over XML views.
Microsoft’s SQL Server is the only one that supports queries over XML views,
but this query support is very limited. This is because queries are specified using
XPath, which is a subset of XQuery. For instance, unlike XQuery, XPath cannot
specify joins. (here I’m talking about the support of the relational database system
of XML views without using any other system in the top of it, and as I mentioned

before, this support is limited, and that is why we are looking on a system that can
fills this gap).
XPERANTO middleware system allows existing relational data to be viewed and
queried as XML, and which works on top of any relational database system.
XPERANTO automatically creates a default XML view, which is a low-level
XML view of the underlying relational database. Users can define their own
views on top of the default views on top of the default view using XQuery. The
main advantage of this approach is that a “standard” XML query language is used
to create and query views. However, if we want to use relational database systems
alone for XML views without XPERANTO, a proprietary language is to be used
to define the initial XML view of the underlying relational database.
High-Level Architecture of XPERANTO
XQuery Query
Query Result
XPERANTO
XQuery to SQL
Converter
Tagger
SQL Query
SQL Result
Relational Database
5. My opinion:
XML views provide a flexible way to view the relational data in XML. Some
relational database systems provide XML views and support queries over these kinds
of views. So if the user doesn’t have to use advanced queries, then there is no need to
use a middleware system on top of the RDBMS. However, we can use a middleware
system like XPERANTO in the top of the RDBMS if it does not support XML views,
or if it can’t execute complex XML queries over the views that the RDBMS provides.
The middleware takes care of everything from creating the views to executing the
XML queries and giving the result as XML document. I think that there is a drawback
of using the middleware system, because the user has to do some plumbing to make
the middleware system works on top of the RDBMS. So, we should wait until the
RDBMS companies provide a full support of the XML views in order to get rid of
these middleware systems, and this is my guess to what will happen.