Welcome!

XML Authors: Trevor Parsons, Liz McMillan, Elizabeth White, Carmen Gonzalez, Mike Kavis

Related Topics: XML

XML: Article

Generating XML from Relational Database Tables

This Emerging Standard Enables to Generate XML Fragments from Relational Data

This article looks in detail at how to generate XML data from your relational database. Although the examples were run on Oracle, very little of the code is Oracle specific. You can easily use all the ideas and examples presented here in other relational databases. We did this project at University of Massachusetts Boston as part of the Electronic Field Guide (EFG) project.

XML is the de facto standard for data exchange. It's simple, Unicode based, and platform independent. XML is a metadata language; it contains information about the data. All these features make it an attractive standard for exchanging data.

Why Generate XML from Relational Data?
Today most data (80% or more), is stored in relational databases such as Oracle, DB2, SQL Server 2000, and others. The Internet and Web services are present in our daily lives. A tremendous amount of data is transferred over the Internet between businesses. Data transfers may happen within a company, between different branches, or between different companies and individuals. No matter which of these situations applies to you, it's very likely that you will be asked to ship your data in XML, or that you are already doing it. Relational Database Management Systems (RDBMS) are still the best way to hold data in bulk. In the following paragraph we will give some information about our project and related resources that we used to provide relational data in XML.

Our Project
Our XML project is part of the Electronic Field Guild (EFG). The EFG project is an object-oriented, Web-based database for the identification of species and recording of ecological observations. With funding from the National Science Foundation, this project is the result of the collaborative efforts between the Departments of Computer Science and Biology at the University of Massachusetts Boston. EFG queries the Integrated Taxonomic Information System (ITIS). On each query we only get a very small part of the ITIS database. XML can describe exactly what data it is delivering, and thus is the preferred format for the query results.

There are three ITIS branches: Canadian, Mexican, and U.S. U.S. ITIS provides the whole data in bulk (about 85MB), but, like many other organizations, it has been slow to join the modern trend to answer queries on it in XML format. Canadian ITIS provides query results in XML, but due to networking delays and some other problems, getting the result takes a long time. We decided to bring the whole database home and return the results of queries in XML format. We are using an RDBMS to store the bulk data. In the following paragraphs we will explain how to generate XML from this relational data.

SQL/XML
This emerging standard enables us to generate XML fragments from relational data. Oracle and many other relational databases support the following standard SQL/XML functions. We will explain most of the SQL/XML functions, XMLElement, XMLAttributes, XMLForest, XMLAgg, and give an example for each.

XMLElement
As its name implies, this function generates an XML element. It takes an element name, an optional collection of attributes for the element, and zero or more arguments that make up the element content and returns an instance of type XMLType.

Table 1 is the schema of the "experts" table from our database. All the simple examples are related to this table, and Oracle was used for most cases.

Table 2 shows some of the data from the experts table.

in ORACLE:
SQL> SELECT
XMLELEMENT("name", expert)
FROM experts
WHERE expert_id BETWEEN 4 and 10;

in DB2:
SELECT
XML2CLOB( XMLELEMENT(NAME "name", expert) )
FROM experts
WHERE expert_id BETWEEN 4 and 10

XMLELEMENT("NAME",EXPERT)
--------------------------------------
<name>Stotler, Raymond E.</name>
<name>Alfred L. Gardner</name>
<name>Steve J. Upton</name>
<name>Wayne Starnes</name>
<name>Lynne Parenti</name>

Here, name is the tag name; expert is the corresponding column name in the experts table. This query obtains the expert column value from the experts table and puts <name> and </name> tags around it.

XMLAttributes
This function simply creates attributes for an element. Listing 1 shows how it works.

Each name element has an id attribute. Each id attribute is obtained from the expert_id column value of the experts table. The XMLAttributes clause isn't used alone; it's used inside an XMLElement function. This makes sense since, in XML, an attribute is a name-value pair attached to the associated element's start tag.

XMLForest
The XMLForest() function produces a forest of XML elements from the given list of arguments. In other words, it produces many XML elements at a time.

In Listing 2, expert is the root element. The id, name, and info elements are children of expert element. The XMLForest function created three elements: the id element corresponds to expert_id column value; the name element corresponds to expert column value; and the info element corresponds to the exp_comment column value in experts table. Naming the elements in this function is accomplished by using AS keyword. For the first element it is expert_id as "id".

What if you don't specify an id (id = 6 in this example)? In this case you will have the appropriate XML fragment for each row.

XMLAgg
XMLAgg() is an aggregate function that produces a forest of XML elements derived from a set of rows.

In Listing 3, there are five name elements as children of the experts root element. Without using XMLAgg() function it's impossible to have many name elements in a single XML document. If you don't use the XMLAgg() function and instead submit the query, you will get Listing 4.

That's not what we wanted. For this output, each experts element has only one name child element, and there are many experts elements. To combine information from multiple rows of the table we need to use the XMLAgg() function.

Creating hierarchical data is easy. Listing 5 shows you how to do this. You can use this idea in similar queries. This example involves the taxonomic_units table, which stores data about species, phyla, etc., i.e. nodes in the tree of life. Note the select within the select, to run through the inner loop.

I hope these examples have convinced you that it's possible to generate XML that obeys any XML Schema or DTD. You can use these functions, nested in each other, to display any kind of parent-child relation. Another such query could display all the experts for each taxon.

This is easy! You can start using these functions as soon as you finish reading this article. Being flexible adds value to this approach. An important point that's worth mentioning is that all the work is done inside the database by using the SQL/XML functions. In the Oracle case, all the work is done in the XML DB Engine. This is much faster and more efficient than doing the work outside the database. Another approach would be to get the data from the database and tag it outside the database for creating XML. This is possible but less efficient, more time consuming. The following are more advanced topics, such as XMLType View and XML Schema validation.

XMLType View
A view is a table that results from a subquery, but which has its own name and can be treated in most ways as if it were an ordinary table. Thus a view table is a logical window on selected data from the base tables and other views. XMLType views wrap the relational data in XML formats. In other words you can have a virtual XML file over your relational data inside the database. We can treat this new view as XML, and use XML specific operations on it, such as XPath and XQuery. These types of operations will be converted to corresponding SQL; this is known as query rewrite. You can create this view by using SQL/XML functions. Our SQL, which generates an XMLType view is shown in the source code, create.sql, online at www.syscon.com/xmlj/sourcec.cfm. Below is a simpler example, for better understanding.


SQL>  CREATE OR REPLACE VIEW expert_view of XMLTYPE WITH OBJECT ID
(EXTRACT(sys_nc_rowinfo$,'/experts/expert/@id').getnumberval()) AS 
SELECT XMLELEMENT("experts",
XMLAGG( XMLELEMENT("expert", XMLATTRIBUTES(expert_id as "id"), expert)))
  	 FROM experts
WHERE expert_id BETWEEN 4 and 10;

View created.

This is how the view looks when you query it. This XMLType view is created over the experts table. The data displayed comes from the underlying relational table.


SQL> SELECT * FROM expert_view;

SYS_NC_ROWINFO$

<experts>
  <expert id="4">Stotler, Raymond E.</expert>
  <expert id="5">Alfred L. Gardner</expert>
  <expert id="6">Steve J. Upton</expert>
  <expert id="8">Wayne Starnes</expert>
  <expert id="10">Lynne Parenti</expert>
</experts>

It's possible to use XPath expressions on this view. Following is a query that has an XPath expression. This query extracts the value of an expert element that has an id attribute equal to 4.

SQL> SELECT extract(value(x), '/experts/expert[@id=4]') FROM expert_view x;
EXTRACT(VALUE(X),'/EXPERTS/EXPERT[@ID=4]')

<expert id="4">Stotler, Raymond E.</expert>

First Step: Register an XSD
You can validate an XMLType instance against XML Schema Documentation (XSD) in Oracle. First, register the XSD in Oracle. Next, call a function that does the validation. There are several functions in Oracle for accomplishing this. Listing 6 is the only one shown, for simplicity.

This will register the XSD and name it as expert_view.xsd. You can name it anything you want of course.

Second Step: Validation
This section introduces isSchemaValid (argumet1, argument2). The first argument is the name of the registered XSD. The second argument is the name of the root element in the specified XSD. XSD can have more than one root. If the XML document (x in this case) is valid, this isSchemaValid returns 1.

SQL> select x.isSchemaValid('expert_view.xsd', 'experts') from expert_view x;

X.ISSCHEMAVALID('EXPERT_VIEW.XSD','EXPERTS')

1

Putting It Together
For our project we generated XML that obeys our XML Schema Documentation (XSD)[itis.xsd]. Our SQL is shown in the source code for this article (available online at www.sys-con.com/xmlj/sourcec.cfm). It will output all the data in XML. As you can see, this query is significantly nested and long, but it's easy to understand (I hope). There are a few more functions in this query that we would like to mention: trim() simply gets rid of the white space in an entry and the to_char() function is used to change the date format. For example, to_char(v.up date_date, 'YYYY-MM-DD') will display the date as something like 2004-04-23. This is somewhat important because the date format in the database doesn't match the date format of the XML Schema.

XML Delivery
In this project we deliver XML through a search server. Using this server, you can query the database by tsn, which is identical to the identification number of a taxonomic unit. e.g. a particular animal or plant. For this part of the project we used:

  1. Sun Enterprise 250 Server: 2 X UltraSPARC-II 400MHz, 2GB memory, SCSI disks
  2. Sun Solaris Operating System 5.8
  3. Oracle 9.2i
  4. JAVA 1.4.2
  5. Java Database Connectivity (JDBC) inside JavaBeans for connecting to and querying the database. We preferred the Oracle thin driver to the oci driver.
  6. JavaServer Pages (JSP) at the user interface level
  7. Tomcat 5.0.12
JSP gets a request from the user, and passes it to a JavaBean. The JavaBean connects to Oracle using JDBC, gets information from the database, and passes it back to the JSP. For better performance:
  • Use Oracle Connection Pool
  • Turn off the auto commit feature
  • Use predefined column types in the select statement.
Conclusion
If you are not already doing so, you will probably be delivering your data in XML format soon. Keep in mind that most data resides in relational databases so generating XML from relational databases will be a very common task. Many of the database vendors, such as Oracle and DB2, provide the SQL/XML function support to make this task easier. SQL Server 2000 adds a new clause, the FOR XML clause, to the SELECT statement, which instructs SQL Server to return the result of a query in XML format.

Based on our experiments, generating XML data inside the database is fast; it took an average of 0.032 seconds per taxon query. Storing your data without the start and end tags is space efficient. Relational databases are very mature about storing, querying, and retrieving data quickly and efficiently. There has been over 20 years of work done in these technologies.

For larger projects, having XMLType views over the relational tables is advantageous. It gives you the chance to query the data using XPath and XQuery besides SQL.

In this article we provided some insight about SQL/XML functions, mentioned the benefits of storing data in relational databases, and explored the benefits of using XMLType views over relational data. For more detailed information about SQL/XML functions, check your database documentation.

References

  • O'Neil, Patrick and Elizabeth. (2000) Database:Principles, Programming, Performance. Morgan Kauffman.
  • and "Electronic Field Guide: An Object-Oriented WWW Database to Identify Species and Record Ecological Observations" www.cs.umb.edu/efg
  • Oracle9i Database Online Documentation http://otn.oracle.com/pls/db92/db92.homepage
  • Appelquist, Daniel K. (2001). XML and SQL: Developing Web Applications. Addison-Wesley.
  • Katz, Howard; and Chamberlin, D.D. (2003) XQuery from the Experts: A Guide to the W3C XML Query Language. Addison-Wesley.
  • More Stories By Selim Mimaroglu

    Selim Mimaroglu is a PhD candidate in computer science at the University of Massachusetts in Boston. He holds an MS in computer science from that school and has a BS in electrical engineering.

    Comments (0)

    Share your thoughts on this story.

    Add your comment
    You must be signed in to add a comment. Sign-in | Register

    In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


    @ThingsExpo Stories

    ARMONK, N.Y., Nov. 20, 2014 /PRNewswire/ --  IBM (NYSE: IBM) today announced that it is bringing a greater level of control, security and flexibility to cloud-based application development and delivery with a single-tenant version of Bluemix, IBM's platform-as-a-service. The new platform enables developers to build ap...

    The BPM world is going through some evolution or changes where traditional business process management solutions really have nowhere to go in terms of development of the road map. In this demo at 15th Cloud Expo, Kyle Hansen, Director of Professional Services at AgilePoint, shows AgilePoint’s unique approach to dealing with this market circumstance by developing a rapid application composition or development framework.
    "BSQUARE is in the business of selling software solutions for smart connected devices. It's obvious that IoT has moved from being a technology to being a fundamental part of business, and in the last 18 months people have said let's figure out how to do it and let's put some focus on it, " explained Dave Wagstaff, VP & Chief Architect, at BSQUARE Corporation, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4-6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
    The major cloud platforms defy a simple, side-by-side analysis. Each of the major IaaS public-cloud platforms offers their own unique strengths and functionality. Options for on-site private cloud are diverse as well, and must be designed and deployed while taking existing legacy architecture and infrastructure into account. Then the reality is that most enterprises are embarking on a hybrid cloud strategy and programs. In this Power Panel at 15th Cloud Expo (http://www.CloudComputingExpo.com), moderated by Ashar Baig, Research Director, Cloud, at Gigaom Research, Nate Gordon, Director of T...
    The Internet of Things is not new. Historically, smart businesses have used its basic concept of leveraging data to drive better decision making and have capitalized on those insights to realize additional revenue opportunities. So, what has changed to make the Internet of Things one of the hottest topics in tech? In his session at @ThingsExpo, Chris Gray, Director, Embedded and Internet of Things, discussed the underlying factors that are driving the economics of intelligent systems. Discover how hardware commoditization, the ubiquitous nature of connectivity, and the emergence of Big Data a...
    SYS-CON Events announced today that Windstream, a leading provider of advanced network and cloud communications, has been named “Silver Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9–11, 2015, at the Javits Center in New York, NY. Windstream (Nasdaq: WIN), a FORTUNE 500 and S&P 500 company, is a leading provider of advanced network communications, including cloud computing and managed services, to businesses nationwide. The company also offers broadband, phone and digital TV services to consumers primarily in rural areas.
    “In the past year we've seen a lot of stabilization of WebRTC. You can now use it in production with a far greater degree of certainty. A lot of the real developments in the past year have been in things like the data channel, which will enable a whole new type of application," explained Peter Dunkley, Technical Director at Acision, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
    SYS-CON Events announced today that IDenticard will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. IDenticard™ is the security division of Brady Corp (NYSE: BRC), a $1.5 billion manufacturer of identification products. We have small-company values with the strength and stability of a major corporation. IDenticard offers local sales, support and service to our customers across the United States and Canada. Our partner network encompasses some 300 of the world's leading systems integrators and security s...
    DevOps Summit 2015 New York, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that it is now accepting Keynote Proposals. The widespread success of cloud computing is driving the DevOps revolution in enterprise IT. Now as never before, development teams must communicate and collaborate in a dynamic, 24/7/365 environment. There is no time to wait for long development cycles that produce software that is obsolete at launch. DevOps may be disruptive, but it is essential.
    "People are a lot more knowledgeable about APIs now. There are two types of people who work with APIs - IT people who want to use APIs for something internal and the product managers who want to do something outside APIs for people to connect to them," explained Roberto Medrano, Executive Vice President at SOA Software, in this SYS-CON.tv interview at Cloud Expo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
    Nigeria has the largest economy in Africa, at more than US$500 billion, and ranks 23rd in the world. A recent re-evaluation of Nigeria's true economic size doubled the previous estimate, and brought it well ahead of South Africa, which is a member (unlike Nigeria) of the G20 club for political as well as economic reasons. Nigeria's economy can be said to be quite diverse from one point of view, but heavily dependent on oil and gas at the same time. Oil and natural gas account for about 15% of Nigera's overall economy, but traditionally represent more than 90% of the country's exports and as...
    The Internet of Things is a misnomer. That implies that everything is on the Internet, and that simply should not be - especially for things that are blurring the line between medical devices that stimulate like a pacemaker and quantified self-sensors like a pedometer or pulse tracker. The mesh of things that we manage must be segmented into zones of trust for sensing data, transmitting data, receiving command and control administrative changes, and peer-to-peer mesh messaging. In his session at @ThingsExpo, Ryan Bagnulo, Solution Architect / Software Engineer at SOA Software, focused on desi...
    "At our booth we are showing how to provide trust in the Internet of Things. Trust is where everything starts to become secure and trustworthy. Now with the scaling of the Internet of Things it becomes an interesting question – I've heard numbers from 200 billion devices next year up to a trillion in the next 10 to 15 years," explained Johannes Lintzen, Vice President of Sales at Utimaco, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
    "For over 25 years we have been working with a lot of enterprise customers and we have seen how companies create applications. And now that we have moved to cloud computing, mobile, social and the Internet of Things, we see that the market needs a new way of creating applications," stated Jesse Shiah, CEO, President and Co-Founder of AgilePoint Inc., in this SYS-CON.tv interview at 15th Cloud Expo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
    SYS-CON Events announced today that Gridstore™, the leader in hyper-converged infrastructure purpose-built to optimize Microsoft workloads, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Gridstore™ is the leader in hyper-converged infrastructure purpose-built for Microsoft workloads and designed to accelerate applications in virtualized environments. Gridstore’s hyper-converged infrastructure is the industry’s first all flash version of HyperConverged Appliances that include both compute and storag...
    Today’s enterprise is being driven by disruptive competitive and human capital requirements to provide enterprise application access through not only desktops, but also mobile devices. To retrofit existing programs across all these devices using traditional programming methods is very costly and time consuming – often prohibitively so. In his session at @ThingsExpo, Jesse Shiah, CEO, President, and Co-Founder of AgilePoint Inc., discussed how you can create applications that run on all mobile devices as well as laptops and desktops using a visual drag-and-drop application – and eForms-buildi...
    We certainly live in interesting technological times. And no more interesting than the current competing IoT standards for connectivity. Various standards bodies, approaches, and ecosystems are vying for mindshare and positioning for a competitive edge. It is clear that when the dust settles, we will have new protocols, evolved protocols, that will change the way we interact with devices and infrastructure. We will also have evolved web protocols, like HTTP/2, that will be changing the very core of our infrastructures. At the same time, we have old approaches made new again like micro-services...
    Code Halos - aka "digital fingerprints" - are the key organizing principle to understand a) how dumb things become smart and b) how to monetize this dynamic. In his session at @ThingsExpo, Robert Brown, AVP, Center for the Future of Work at Cognizant Technology Solutions, outlined research, analysis and recommendations from his recently published book on this phenomena on the way leading edge organizations like GE and Disney are unlocking the Internet of Things opportunity and what steps your organization should be taking to position itself for the next platform of digital competition.
    The 3rd International Internet of @ThingsExpo, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that its Call for Papers is now open. The Internet of Things (IoT) is the biggest idea since the creation of the Worldwide Web more than 20 years ago.
    As the Internet of Things unfolds, mobile and wearable devices are blurring the line between physical and digital, integrating ever more closely with our interests, our routines, our daily lives. Contextual computing and smart, sensor-equipped spaces bring the potential to walk through a world that recognizes us and responds accordingly. We become continuous transmitters and receivers of data. In his session at @ThingsExpo, Andrew Bolwell, Director of Innovation for HP's Printing and Personal Systems Group, discussed how key attributes of mobile technology – touch input, sensors, social, and ...