XML and The Relational Data Model
<author> By: Soid Quintero </author>
Overview
Define XML
XML model vs. relational model
XML Documents
Xpath
XQuery
Going from XML to the relational model
What is XML?
XML stands for Extensible Markup Language
It is a metalanguage used to represent and manipulate data elements
Similar to HTML in structure, but XML is concerned with the description
and representation of data, rather than with the way it is displayed.
XML is derived from Standard Generalized Markup Language (SGML)
XML data is used to create XML documents.
Why XML for databases?
One of the main reasons that XML was developed was to allow the
exchange of semi-structured documents, like invoices, order forms,
applications, etc., over the internet.
Using a database system to store XML documents allows users to be
able to better access information.
XML is also very flexible
Data is maintained in a self-describing format to accommodate a
variety of ever-evolving business needs.
What is an XML Database?
Simply a database that stored XML Documents
There are two major types of XML databases:
XML-enabled. These map all XML to a traditional database (such as a
database), accepting XML as input and rendering XML as output.
Native XML (NXD) The internal model of such databases depends on
XML and uses XML documents as the fundamental unit of storage.
XML Model VS. Relational Model?
XML data is hierarchical
XML data is self-describing
XML data has inherent ordering
 An XML database contains
collections
relational data is represented in a
model of logical relationships.
relational data is not selfdescribing.
Relational data does not have
inherent ordering.
A relational database contains
tables
Relational Model
Order of rows is not guaranteed unless the ORDER clause is
used in one or more columns
Relations (tables)
Data is represented in n-ary relations.
Has a domain that represents a set of values
Attributes (columns)
Strict schema
Restrictive
The strict schema insures data integrity
XML Model
The XML Model is hierarchical format
Data is represented in trees structures
There's nodes
Relationships between the node
The schema provides flexibility
Easily modified format
multiple elements represented in a hierarchy,
including a root“Comments” element
and one or more individual “Comment”
elements pertaining to a given item.
XML Document Rules
XML documents must be well formated, meaning that every opening tag
needs a closing tag (ex. <Student> </Student> )
It allows users to define their own tags (unlike HTML)
The XML tags need to be properly nested.
The XML and xml tags are reserved for XML tags only.
You can use <-- XXX --> symbols for comments
XML is case sensitive so, <Student> is not the same as <STUDENT>
Two types of XML documents commonly used, Document Type Definition
(DTD) or an XML Schema Definitions (XSD)
What is a Document Type Definition?
DTDs can be declared inline of the XML code or can reference an
external file
It provides the composition of the database's logical model and
defines the syntax rules or valid tags for each type of XML document.
A DTD is a file that has a .dtd extension.
This file describes XML elements.
Example of a DTD on next slide.......
Inline DTD
message.xml
<?xml version="1.0"?>
<!DOCTYPE message [
<!ELEMENT message
(to,from,subject,text)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT subject (#PCDATA)>
<!ELEMENT text (#PCDATA)>
]>
<message>
<to>Dave</to>
<from>Susan</from>
<subject>Reminder</subject>
<text>Don't forget to buy milk on
the way home.</text>
</message>
External DTD
message.xml
<?xml version="1.0"?>
<!DOCTYPE message SYSTEM
"message.dtd">
<message>
<to>Dave</to>
<from>Susan</from>
<subject>Reminder</subject>
<text>Don't forget to buy milk
on the way home.</text>
</message>
message.dtd
<?xml version="1.0"?>
<!ELEMENT message
(to,from,subject,text)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT subject (#PCDATA)>
<!ELEMENT text (#PCDATA)>
What is an XML Schema Definition?
The XML Schema is an advance definition language that is used to
describe the structure (elements, data types, relationships types,
ranges, and default values)
It is an alternative to DTD
Since data types are allowed data validation is possible and easier to
do.
A XSD file has a .xsd extension
Example of a XSD on next slide.......
<?xml version="1.0"?>
<xs:schema
xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://www.w3schools.com"
xmlns="http://www.w3schools.com"
elementFormDefault="qualified">
<xs:element name="note">
<xs:complexType>
<xs:sequence>
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="heading" type="xs:string"/>
<xs:element name="body" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
How do you access XML data?
There are several languages used to access XML data from XML
Documents, some are:
XPath
XQuery
XML-QL
XQL
etc....
XPath
Xpath is a language used to extract parts of an XML document.
XPath uses path expressions to navigate in XML documents
Xpath has 7 kinds of nodes:
Element
Attribute
Text
Namespace
Processing-Instruction
Comment
Document(root)
Examples of XPath
<?xml version="1.0" encoding="ISO-8859-1"?>
<bookstore>
Attribute node
Document node
<book>
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
Element node
</bookstore>
Example of an XPath expressions:
/bookstore
Will select the root element bookstore
/bookstore/book Selects all book elements that are children of bookstore
XQuery
XQuery is a language for finding and extracting elements and
attributes from XML documents.
XQuery for XML is like SQL for databases
XQuery is built on XPath expressions
XQuery is supported by all the major database engines (IBM,
Oracle, Microsoft, etc.)
Example of XQuery
<?xml version="1.0" encoding="ISO-88591"?>
<bookstore>
<book category="COOKING">
<title lang="en">Everyday
Italian</title>
<author>Giada De Laurentiis</author>
<year>2005</year>
<price>30.00</price>
</book>
</bookstore>
Example of an XQuery expressions:
doc("books.xml")/bookstore/book/title
Will return:
<title lang="en">Everyday Italian</title>
The XQuery FLWOR
for: Iterates through a sequence, bind variable to items
let: binds a variable to a sequence
where:eliminates items of the iteration
order by: reorders items of the iteration
return: constructs query results
Example of a XQuery expression:
for $x in doc("books.xml")/bookstore/book
where $x/price>29
return $x/title
Will return:
<title lang="en">Everyday Italian</title>
Going from XML Model to
Relational Model
XML documents can be decomposed into a relational table.
Those decomposed XML documents can be made into relational tables
and published to an XML document( they might differ from the original)
During the decomposing process, the XML document loses most of it
structure in order to map into the relational table; not all the tags are
stored in the relational tables.
Example of an XML document
<ORDER>
<ORDER_ID=’83492’ CUST_ID=’93457’>
<ITEM>
<PROD_ID>94872</PROD_ID>
<PROD_NAME>PEN</PROD_NAME>
<PRICE>19.95</PRICE>
<QUANTITY>30</QUANTITY>
</ITEM>
<ITEM>
<PROD_ID>94866</PROD_ID>
<PROD_NAME>BINDER</PROD_NAME>
<PRICE>7.95</PRICE>
<QUANTITY>26</QUANTITY>
</ITEM>
<ITEM>
<PROD_ID>92219</PROD_ID>
<PROD_NAME>LABELS</PROD_NAME>
<PRICE>12.95</PRICE>
<QUANTITY>250</QUANTITY>
</ITEM>
</ORDER>
XML document decomposed into
a relation
Order
Items
Another Example.....
Sales Order
<SalesOrder Number="123">
<OrderDate>2003-07-28</OrderDate>
<CustomerNumber>456</CustomerNumber>
<Item Number="1">
<PartNumber>XY47</PartNumber>
<Quantity>14</Quantity>
<Price>16.80</Price>
</Item>
<Item Number="2">
<PartNumber>B987</PartNumber>
<Quantity>6</Quantity>
<Price>2.34</Price>
</Item>
</SalesOrder>
Items table
Hybrid DBMS
There are DBMS that allow the use of both the relational model and
the XML data model, a so call “Hybrid” model.
An example of such a DBMS is IBMs DB2.
Architecture of DB2
References
(1) “Comparison of XML Model and the Relational Model.” DB2 Version 9 for Linux, UNIX,
and Windows.10 April. 2007
<http://publib.boulder.ibm.com/infocenter/db2luw/v9/topic/com.ibm.db2.udb.apdv.embed.
doc/doc/c0023811.htm>.
(2) W3Schools. <http://www.w3schools.com/xpath/default.asp>
(3) W3Schools. <http://www.w3schools.com/xquery/default.asp>
(4) Coss Rafael. “DBS 9 pureXML”. IMSS_DB2pureXML_v3.ppt.
(5) Coss, Rafael. “XML L2 Skills Transfer - template”. Jan 14, 2005. XML_L2~1.ppt
(6) Bourret, Ronald. XML “DatabaseProducts“. Ronald Bourret Consulting, writing, and
research in XML and database. March 13, 2007.
<http://www.rpbourret.com/xml/XMLDatabaseProds.htm#xmlanddatabases>
(7) W3Schools. <http://www.w3schools.com/dtd/default.asp>
(8)Steegmans, Bart. Bourret, Ronald. Olivier, Guvennet.” XML for DB2
Information Integration”. IBM Redbooks
.<http://www.redbooks.ibm.com/redbooks/SG246994/wwhelp/wwhimpl/java/ht
ml/wwhelp.htm>