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AN EXPLORATION OF
XML IN DATABASE
MANAGEMENT SYSTEMS
By
Dare Obasanjo
Introduction: XML and Data
XML stands for eXtensible Markup Language. XML is a meta-markup language
developed by the World Wide Web Consortium(W3C) to deal with a number of the
shortcomings of HTML. As more and more functionality was added to HTML to account
for the diverse needs of users of the Web, the language began to grow increasingly
complex and unwieldy. The need for a way to create domain-specific markup languages
that did not contain all the cruft of HTML became increasingly necessary and XML was
born.
The main difference between HTML and XML is that whereas in HTML the semantics
and syntax of tags is fixed, in XML the author of the document is free to create tags
whose syntax and semantics are specific to the target application. Also the semantics of a
tag is not tied down but is instead dependent on the context of the application that
processes the document. The other significant differences between HTML and XML is
that the an XML document must be well-formed.
Although the original purpose of XML was as a way to mark up content, it became clear
that XML also provided a way to describe structured data thus making it important as a
data storage and interchange format. XML provides many advantages as a data format
over others, including:
1. Built in support for internationalization due to the fact that it utilizes unicode.
2. Platform independence (for instance, no need to worry about endianess).
3. Human readable format makes it easier for developers to locate and fix errors than
with previous data storage formats.
4. Extensibility in a manner that allows developers to add extra information to a
format without breaking applications that where based on older versions of the
format.
5. Large number of off-the-shelf tools for processing XML documents already exist.
The world of traditional data storage and XML have never been closer together. To better
understand how data storage and retrievel works in an XML world, this paper will first
discuss the past, present, and future of structuring XML documents. Then we will delve
into the languages that add the ability to query an XML document similar to a traditional
data store. This will be followed by an exploration of how the most popular RDBMSs
have recognized the importance of this new data storage format and have integrated XML
into their latest releases. Finally the rise of new data storage and retrieval systems
specifically designed for handling XML will be shown.
Structuring XML: DTDs and XML Schemas
Since XML is a way to describe structured data there should be a means to specify the
structure of an XML document. Document Type Definitions (DTDs) and XML Schemas
are different mechanisms that are used to specify valid elements that can occur in a
document, the order in which they can occur and constrain certain aspects of these
elements. An XML document that conforms to a DTD or schema is considered to be
valid. Below is listing of the different means of constraining the contents of an XML
document.
SAMPLE XML FRAGMENT
<gatech_student gtnum="gt000x">
<name>George Burdell</name>
<age>21</age>
</gatech_student>
1. Document Type Definitions (DTD): DTDs were the original means of
specifying the structure of an XML document and a holdover from XML's roots
as a subset of the Standardized and General Markup Language(SGML). DTDs
have a different syntax from XML and are used to specify the order and
occurence of elements in an XML document. Below is a DTD for the above XML
fragment.
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DTD FOR SAMPLE XML FRAGMENT
<!ELEMENT
<!ATTLIST
<!ELEMENT
<!ELEMENT
gatech_student (name, age)>
gatech_student gtnum CDATA>
name (#PCDATA)>
age (#PCDATA)>
The DTD specifies that the gatech_student element has two child elements, name
and age, that contain character data as well as a gtnum attribute that contains
character data.
9. XML Data Reduced (XDR): DTDs proved to be inadequate for the needs of
users of XML due to to a number of reasons. The main reasons behind the
criticisms of DTDs were the fact that they used a different syntax than XML and
their non-existent support for datatypes. XDR, a recommendation for XML
schemas, was submitted to the W3C by the Microsoft Corporation as a potential
XML schema standard which but was eventually rejected. XDR tackled some of
the problems of DTDs by being XML based as well as supporting a number of
datatypes analogous to those used in relational database management systems and
popular programming languages. Below is an XML schema, using XDR, for the
above XML fragment.
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XDR FOR SAMPLE XML FRAGMENT
<Schema name="myschema" xmlns="urn:schemas-microsoft-com:xmldata"
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xmlns:dt="urn:schemas-microsoftcom:datatypes">
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<ElementType name="age" dt:type="ui1" />
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<ElementType name="name" dt:type="string" />
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<AttributeType name="gtnum" dt:type="string" />
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<ElementType name="gatech_student" order="seq">
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<element type="name" minOccurs="1" maxOccurs="1"/>
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<element type="age" minOccurs="1" maxOccurs="1"/>
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<attribute type="gtnum" />
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</ElementType>
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</Schema>
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The above schema specifies types for a name element that contains a string as its
content, an age element that contains an unsigned integer value of size one byte
(i.e. btw 0 and 255), and a gtnum attribute that is a string value. It also specifies a
gatech_student element that has one occurence each of a name and an age element
in sequence as well as a gtnum attribute.
24. XML Schema Definitions (XSD) : The W3C XML schema recommendation
provides a sophisticated means of describing the structure and constraints on the
content model of XML documents. W3C XML schema support more datatypes
than XDR, allow for the creation of custom data types, and support object
oriented programming concepts like inheritance and polymorphism. Currently
XDR is used more widely than than W3C XML schema but this is primarily
because the XML Schema recommendation is fairly new and will thus take time
to become accepted by the software industry.
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XSD FOR SAMPLE XML FRAGMENT
<schema xmlns="http://www.w3.org/2001/XMLSchema" >
<element name="gatech_student">
<complexType>
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<sequence>
<element name="name" type="string"/>
<element name="age" type="unsignedInt"/>
</sequence>
<attribute name="gtnum">
<simpleType>
<restriction base="string">
<pattern value="gt\d{3}[A-Za-z]{1}"/>
</restriction>
</simpleType>
</attribute>
</complexType>
</element>
</schema>
The above schema specifies a gatech_student complex type (meaning it can have
elements as children) that contains a name and an age element in sequence as well
as a gtnum attribute. The name element has to have a string as content, the age
attribute has an unsigned integer value while the gtnum element has to be
matched by a regular expression that matches the letters "gt" followed by 3 digits
and a letter.
The above examples show that DTDs give the least control over how one can constrain
and structure data within an XML document while W3C XML schemas give the most.
XML Querying: XPath and XQuery
It is sometimes necessary to extract subsets of the data stored within an XML document.
A number of languages have been created for querying XML documents including Lorel,
Quilt, UnQL, XDuce, XML-QL, XPath, XQL, XQuery and YaTL. Since XPath is
already a W3C recommendation while XQuery is on its way to becoming one, the focus
of this section will be on both these languages. Both languages can be used to retrieve
and manipulate data from an XML document.
1. XML Path Language (XPath): XPath is a language for addressing parts of an
XML document that utilizes a syntax that resembles hierarchical paths used to
address parts of a filesystem or URL. XPath also supports the use of functions for
interacting with the selected data from the document. It provides functions for the
accessing information about document nodes as well as for the manipulation of
strings, numbers and booleans. XPath is extensible with regards to functions
which allows developers to add functions that manipulate the data retrieved by an
XPath query to the library of functions available by default. XPath uses a
compact, non-XML syntax in order to facilitate the use of XPath within URIs and
XML attribute values (this is important for other W3C recommendations like
XML schema and XSLT that use XPath within attributes).
XPath operates on the abstract, logical structure of an XML document, rather than
its surface syntax. XPath is designed to operate on a single XML document which
it views as a tree of nodes and the values returned by an XPath query are
considered conceptually to be nodes. The types of nodes that exist in the XPath
data model of a document are text nodes, element nodes, attribute nodes, root
nodes, namespace nodes, processing instruction nodes, and comment nodes.
Sample XPath Queries Against Sample XML Fragment
a.
/gatech_student/name
Selects all name elements that are children of the root element
gatech_student.
b.
//age
Selects all age elements in the document.
c.
/gatech_student/*
Selects all child elements of the root element gatech_student.
d.
/gatech_student[@gtnum]
Selects all gtnum attributes of the gatech_student elements in the
document.
e.
//*[name()='age']
Selects all elements that are named "age".
f.
/gatech_student/age/ancestor::*
Selects all ancestors of all the age elements that are children of the
gatech_student element (which should select the gatech_student element).
2.
3. XML Query Language (XQuery): XQuery is an attempt to provide a query
language that provides the same breadth of functionality and underlying
formalism as SQL does for relational databases. XQuery is a functional language
where each query is an expression. XQuery expressions fall into seven broad
types; path expressions, element constructors, FLWR expressions, expressions
involving operators and functions, conditional expressions, quantified expressions
or expressions that test or modify datatypes. The syntax and semantics of the
different kinds of XQuery expressions vary significantly which is a testament to
the numerous influences in the design of XQuery.
XQuery has a sophisticated type system based on XML schema datatypes and
supports the manipulation of the document nodes unlike XPath. Also the data
model of XQuery is not only designed to operate on a single XML document but
also a well-formed fragment of a document, a sequence of documents, or a
sequence of document fragments.
W3C is also working towards creating an alternate version of XQuery that has the
same semantics but uses XML based syntax instead called XQueryX.
Sample XQuery Queries and Expressions Taken From W3C Working Draft
a. path expressions: XQuery supports path expressions that are a superset of
those currently being proposed for the next version of XPath.
i.
//emp[name="Fred"]/salary * 12
From a document that contains employees and their monthly
salaries, extract the annual salary of the employee named "Fred".
ii.
document("zoo.xml")//chapter[2 TO 5]//figure
Find all the figures in chapters 2 through 5 of the document named
"zoo.xml."
b. element constructors: In some situations, it is necessary for a query to
create or generate elements. Such elements can be embeded directly into a
query in an expression called an element constructor.
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<emp empid = {$id}>
{$name}
{$job}
</emp>
Generate an <emp> element that has an "empid" attribute. The
value of the attribute and the content of the element are specified
by variables that are bound in other parts of the query.
c. FLWR expressions: A FLWR (pronounced "flower") expression is a
query construct composed of FOR, LET, WHERE, and a RETURN
clauses. A FOR clause is an iteration construct that binds a variable to a
sequence of values returned by a query (typically a path expression). A
LET clause similarly binds variables to values but instead of a series of
bindings only one occurs similar to an assignment statement in a
programming language. A WHERE clause contains one or more
predicates that are used on the nodes returned by preceding LET or FOR
clauses. The RETURN clause generates the output of the FLWR
expression, which may be any sequence of nodes or primitive values. The
RETURN clause is executed once for each node returned by the FOR and
LET clauses that passes the WHERE clause. The results of these multiple
executions is concatenated and returned as the result of the expression.
i.
FOR $b IN document("bib.xml")//book
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WHERE $b/publisher = "Morgan Kaufmann"
AND $b/year = "1998"
RETURN $b/title
List the titles of books published by Morgan Kaufmann in 1998.
<big_publishers>
{
FOR $p IN
distinct(document("bib.xml")//publisher)
LET $b :=
document("bib.xml")//book[publisher = $p]
WHERE count($b) > 100
RETURN $p
}
</big_publishers>
List the publishers who have published more than 100 books.
d. conditional expressions: A conditional expression evaluates a test
expression and then returns one of two result expressions. If the value of
the test expression is true, the value of the first result expression is
returned otherwise, the value of the second result expression is returned.
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FOR $h IN //holding
RETURN
<holding>
{$h/title,
IF ($h/@type = "Journal")
THEN $h/editor
ELSE $h/author
}
</holding>
SORTBY (title)
Make a list of holdings, ordered by title. For journals, include the
editor, and for all other holdings, include the author.
e. quantified expressions: XQuery has constructs that are equivalent to
quantifiers used in mathematics and logic. The SOME clause is an
existential quantifier used for testing to see if a series of values contains at
least one node that satisfies a predicate. The EVERY clause is a universal
quantifier used to test to see if all nodes in a series of values satisfy a
predicate.
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FOR $b IN //book
WHERE SOME $p IN $b//para SATISFIES
(contains($p, "sailing") AND contains($p,
"windsurfing"))
RETURN $b/title
Find titles of books in which both sailing and windsurfing are
mentioned in the same paragraph.
FOR $b IN //book
viii.
ix.
x.
xi.
xii.
WHERE EVERY $p IN $b//para SATISFIES
contains($p, "sailing")
RETURN $b/title
Find titles of books in where sailing is mentioned in every
paragraph.
f. expressions involving user defined functions: Besides providing a core
library of functions similar to those in XPath, XQuery also allows user
defined functions to be used to extend the core function library.
i.
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NAMESPACE xsd = "http://www.w3.org/2001/XMLSchema"
DEFINE FUNCTION depth($e) RETURNS xsd:integer
{
# An empty element has depth 1
# Otherwise, add 1 to max depth of children
IF (empty($e/*)) THEN 1
ELSE max(depth($e/*)) + 1
}
depth(document("partlist.xml"))
Find the maximum depth of the document named "partlist.xml."
XML and Databases
As was mentioned in the introduction, there is a dichotomy in how XML is used
in industry. On one hand there is the document-centric model of XML where
XML is typically used as a means to creating semi-structured documents with
irregular content that are meant for human consumption. An example of
document-centric usage of XML is XHTML which is the XML based successor to
HTML.
SAMPLE XHTML DOCUMENT
<html xmlns ="http://www.w3.org/1999/xhtml">
<head>
<title>Sample Web Page</title>
</head>
<body>
<h1>My Sample Web Page</h1>
<p> All XHTML documents must be well-formed and valid.
</p>
<img src="http://www.example.com/sample.jpg" height ="50"
width = "25"/>
<br />
<br />
</body>
</html>
The other primary usage of XML is in a data-centric model. In a data-centric
model, XML is used as a storage or interchange format for data that is structured,
appears in a regular order and is most likely to be machine processed instead of
read by a human. In a data-centric model, the fact that the data is stored or
transferred as XML is typically incidental since it could be stored or transferred in
a number of other formats which may or may not be better suited for the task
depending on the data and how it is used. An example of a data-centric usage of
XML is SOAP. SOAP is an XML based protocol used for exchanging
information in a decentralized, distributed environment. A SOAP message
consists of three parts: an envelope that defines a framework for describing what
is in a message and how to process it, a set of encoding rules for expressing
instances of application-defined datatypes, and a convention for representing
remote procedure calls and responses.
SAMPLE SOAP MESSAGE TAKEN FROM W3C SOAP RECOMMENDATION
<SOAP-ENV:Envelope xmlns:SOAPENV="http://schemas.xmlsoap.org/soap/envelope/"
SOAPENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
<SOAP-ENV:Body>
<m:GetLastTradePrice xmlns:m="Some-URI">
<symbol>DIS</symbol>
</m:GetLastTradePrice>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>
In both models where XML is used, it is sometimes necessary to store the XML in
some sort of repository or database that allows for more sophisticated storage and
retrieval of the data especially if the XML is to be accessed by multiple users.
Below is a description of storage options based on what model of XML usage is
required.
7. Data-centric model: In a data-centric model where data is stored in a
relational database or similar repository; one may want to extract data
from a database as XML, store XML into a database or both. For
situations where one only needs to extract XML from the database one
may use a middleware application or component that retrieves data from
the database and returns it as XML. Middleware components that
transform relational data to XML and back vary widely in the
functionality they provide and how they provide it. For instance,
Microsoft's ADO.NET provides XML integration to such a degree that
results from queries on XML documents or SQL databases can be
accessed identically via the same API. Some like Merant's jxTransformer
require the user to specify how the results of a SQL query should be
converted to XML via a custom query while others like IBM's Database
DOM require the user to create a template file that contains the SQL to
XML mappings for the query to be performed. Another approach is the
one taken by DB2XML where a default mapping of SQL results to XML
data exists that cannot be altered by the user. Middleware components also
vary in how the sophistication of their user interface which may vary from
practically non-existent (interaction done via programmatically using
APIs) to interaction being via a sophisticated graphical user interfaces.
The alternative to using middleware components to retrieve or store XML
in a database is to use an XML-enabled database that understands how to
convert relational data to XML and back. Currently, the Big 3 relational
database products all support retrieving and storing XML in one form or
another. IBM's DB2 uses the DB2 XML Extender. The DB2 extender
gives one the option to store an entire XML document and its DTD as a
user-defined column [of type XMLCLOB,XMLVARCHAR or XMLFile]
or to shred the document into multiple tables and columns. XML
documents can then be queried with syntax that is compliant with W3C
XPath recommendation. Updating of XML data is also possible using
stored procedures.
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SAMPLE DB2 XML EXTENDER TABLE AND QUERY
TABLE mail_user
user_name
VARCHAR(20) NOT NULL PRIMARY KEY
passwd
VARCHAR(10)
mailbox
XMLVARCHAR
SELECT user_name FROM mail_user WHERE
extractVarchar(mailbox,"/Mailbox/Inbox/Email/Subject") LIKE
"%XML%"
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The above query returns the names of all the users
that have any email in their inbox that
18.
contains the string "XML" in its subject. To improve
the performance of the XPath query it is
19.
necessary to index the mailbox XMLVARCHAR.
Oracle has completely integrated XML into it's Oracle 9i database as well
as the rest of its family of products. XML documents can be stored as
whole documents in user-defined columns [of type XMLType or
CLOB/BLOB] where they can be extracted using XMLType functions
such as Extract() or they can be stored as decomposed XML documents
that are stored in object relational form which can be recontituted using
the XML SQL Utility (XSU) or SQL functions and packages. For
searching XML, Oracle provides Oracle Text which can be used to index
and search XML stored in VARCHAR2 or BLOB variables within a table
via the CONTAINS and WITHIN operators used in collusion with SQL
SELECT queries. XMLType columns can be queried by selecting them
through a programming interface (e.g. SQL, PL/SQL, C, or Java), by
querying them directly and using extract() and/or existsNode() or by using
Oracle Text operators to query the XML content. The extract() and
existsNode() functions uses XPath expressions for querying XML data.
Oracle 9i also allows one to create relational views on XML documents
stored in XMLType columns which can then be queried using SQL. The
columns in the table are mapped to XPath expressions that query the
document in the XMLType column.
SAMPLE ORACLE 9i TABLE AND QUERY
CREATE TABLE mail_user(
user_name VARCHAR2(20),
passwd
VARCHAR2(10),
mailbox
SYS.XMLTYPE );
SELECT user_name FROM mail_user m WHERE
m.mailbox.extract('/Mailbox/Inbox/Email/Subject/text()').ge
tStringVal() like '%XML%'
The above query returns the names of all the users that
have any email in their inbox that
contains the string "XML" in its subject. To improve
the performance of the XPath query it is
necessary to index the mailbox XMLType.
Microsoft's SQL Server 2000 also supports XML operations being
performed on relational data . XML data can be retrieved from relational
tables using the FOR XML clause. The FOR XML clause has three
modes: RAW, AUTO and EXPLICIT. RAW mode sends each row of data
in the resultset back as a XML element named "row" and with each
column being an attribute of the "row" element. AUTO mode returns
query results in a nested XML tree where each element returned is named
after the table it was extracted from and each column is an attribute of the
returned elements. The hierarchy is determined based on the order of the
tables identified by the columns of the SELECT statement. With
EXPLICIT mode the hierarchy of the XML returned is completely
controlled by the query which can be rather complex. SQL Server also
provides the OPENXML clause which to provide a relational view on
XML data. OPENXML allows XML documents placed in memory to be
used as parameters to SQL statements or stored procedures. Thus
OPENXML is used to query data from XML, join XML data with existing
relational tables, and insert XML data into the database by "shredding" it
into tables. Also W3C XML schema to can be used to provide mappings
between XML and relational structures. These mappings are called XML
views and allow relational data in tables to be viewed as XML which can
be queried using XPath.
As can be seen from the above descriptions, there is currently no standard
way to access XML from relational databases. This may change with the
development of the SQL/XML standard currently being developed by the
SQLX group.
20. Document-centric model: Content management systems are typically the
tool of choice when considering storing, updating and retrieving various
XML documents in a shared repository. A content management system
typically consists of a repository that stores a variety of XML documents,
an editor and an engine that provides one or more of the following
features:






version, revison and access control
ability to reuse documents in different formats
collaboration
web publishing facilities
support for a variety of text editors (e.g. Microsoft Word, Adobe
Framemaker, etc)
indexing and search capabilities
Content management systems have been primarily of benefit for workflow
management in corporate environments where information sharing is vital
and as a way to manage the creation of web content in a modular fashion
allowing web developers and content creators to perform their tasks with
less interdependence than exists in a traditional web authoring
environment. Examples of XML based content management systems are
SyCOMAX, Content@, Frontier, Entrepid, XDisect, and SiberSafe.
21. Hybrid model: In situations where both documentric-centric and datacentric models of XML usage will occur, the best data storage choice is
usually a native XML database. What actually constitutes a native XML
database has been a topic of some debate in various fora which has been
compounded by the blurred lines that many see between XML-enabled
databases, XML query engines, XML servers and native XML databases.
The most coherrent definition so far is one that was reached by consensus
amongst members of the XML:DB mailing list which defines a native
XML database as a database that has an XML document as its
fundamental unit of (logical) storage and defines a (logical) model for an
XML document, as opposed to the data in that document, and stores and
retrieves documents according to that model. At a minimum, the model
must include elements, attributes, PCDATA, and document order.
Described below are two examples of native XML databases with the
intent of showing the breadth of functionality and variety that can be
expected in the native XML database arena.
Tamino is a native XML database management system developed by
Software AG. Tamino is a relatively mature application, currently at
version 2.3.1, that provides the means to store & retrieve XML
documents, store & retrieve relational data, as well as interface with
external applications and data sources. Tamino has a web based
administration interface similar to that used by the major relational
database management systems and includes GUI tools for interacting with
the database and editting schemas.
Schemas in Tamino are DTD-based and are used primarily as a way to
describe how the XML data should be indexed. When storing XML
documents in Tamino; one can specify a pre-existing DTD which is then
converted to a Tamino schema, store a well-formed XML document
without a schema which means that default indexing ensues or a schema
can be created from scratch for the XML document being stored. A
secondary usage of schemas is for specifying the datatypes in XML
documents. The main advantage of using datatypes in Tamino is to enable
type based operations within queries (e.g. numeric comparisons). The
query language used by Tamino is based on XPath and is called X-Query
(not to be confused with the W3C XQuery).
Tamino also ships with a relational database management system which is
called the SQL Engine. Schemas can be used to creating mappings from
SQL to XML which then allow for the storage or retrieval of XML data
from relational database sources either internal (i.e. the SQL Engine) or
external. Schemas can also be used to represent joins across different
document types. Joins allow for queries to be performed on XML
documents with differing schemas. Future versions of Tamino are
supposed to eliminate the need to specify joins up front in a schema and
instead should allow for such joins to be done dynamically from a query.
Tamino provides APIs for accessing the XML store in both Java and
Microsoft's JScript. C programmers can interact with the SQL engine
using the SQL precompiler that ships with Tamino. Interfaces that allow
ODBC, OLE DB and JDBC clients to communicate with the Tamino SQL
Engine are also available. Finally, Tamino ships with the X-Tensions
framework which allows developers to extend the functionality of Tamino
by using C++ COM objects or Java objects. Tamino operations have
ACID properties (Atomicity, Consistency, Isolation and Durability) via
the support of transactions in its programming interfaces.
dbXML is an Open Source native XML database management system
which is sponsored by the dbXML Group. dbXML is designed for
managing collections of XML documents which are arranged
hierarchically within the system in a manner similar to that of a file
system. Querying the XML documents within the system is done using
XPath and the documents can be indexed to improve query performance.
dbXML is written in Java but supports access from other languages by
exposing a CORBA API thus allowing interaction with any language that
supports a CORBA binding. It also ships with a Java implementation of
the XML:DB XML Database API which is designed to be a vendor neutral
API for XML databases. A number of command line tools for managing
documents and collections are also provided.
dbXML is mostly still in development (version at time of writing was 1.0
beta 2) and does not currently support transactions or the use of schemas
but these features are currently being developed for future versions.
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Conference, San Diego. February 2000. < http://www4.ibm.com/software/data/db2/extenders/xmlext/xmlextbroch.pdf >
Acknowledgements
The following people helped in reviewing and proofreading this paper: Dr. Sham
Navathe, Kimbro Staken, Dmitri Alperovitch, Sam Collins, Omri Gazitt and
Dennis Lu.
© 2001 Dare Obasanjo