Lecture 15 : Introduction to XML
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Introduction to Semistructured
Data and XML
Chapter 27
Database Management Systems, R. Ramakrishnan
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How the Web is Today
HTML documents
• often generated by applications
• consumed by humans only
• easy access: across platforms, across organizations
No application interoperability:
• HTML not understood by applications
• Database technology: client-server
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New Universal Data Exchange
Format: XML
A recommendation from the W3C
XML = data
XML generated by applications
XML consumed by applications
Easy access: across platforms, organizations
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Paradigm Shift on the Web
From documents (HTML) to data (XML)
From information retrieval to data
management
For databases, also a paradigm shift:
• from relational model to semistructured data
• from data processing to data/query translation
• from storage to transport
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HTML
HTML is widely used for formatting and structuring
Web documents.
Designed to describe how a Web browser should
arrange text, images and push-buttons on a page.
Easy to learn, but does not convey structure and
meaning of data in the Web pages.
Fixed tag set.
Text (PCDATA)
Opening tag
<HTML>
<HEAD><TITLE>Welcome to the XML course</TITLE></HEAD>
<BODY>
<H1>Introduction</H1>
<IMG SRC=”dragon.jpeg" WIDTH="200" HEIGHT="150” >
Closing tag
</BODY>
</HTML>
“Bachelor” tag
Attribute name
Database Management Systems, R. Ramakrishnan
Attribute value
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Semistructure data
1.
2.
3.
4.
Information integration: important new application
that motivates what follows.
Semistructured data: a new data model designed to
cope with problems of information integration.
XML (Extensible Markup Language) : a new Web
standard that is essentially semistructured data.
XQUERY: an emerging standard query language
for XML data.
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Information Integration
Problem: related data exists in many places. They talk
about the same things, but differ in model, schema,
conventions (e.g., terminology).
Example: In the real world, every bar has its own
database.
Some may have relations like beer-price; others have
an Microsoft Word file from which the menu is
printed.
Some keep phones of manufacturers but not
addresses.
Some distinguish beers and ales; others do not.
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The Semistructured Data Model
Bib
Object Exchange
Model (OEM)
&o1
complex object
paper
paper
book
references
&o12
&o24
references
author
title
year
&o29
references
author
http
page
author
title publisher
title
author
author
author
&o43
&25
&96
1997
last
firstname
firstname
lastname
&243
“Serge”
“Abiteboul”
“Victor”
lastname
first
&206
“Vianu”
122
133
atomic object
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Characteristics of Semistructured
Data
Missing or additional attributes
Multiple attributes
Different types in different objects
Heterogeneous collections
Self-describing, irregular data, no a priori structure
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Comparison with Relational Data
row
nam e
phone
John
3634
Sue
6343
D ic k
6363
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row
row
name phone name phone name phone
“John” 3634 “Sue” 6343 “Dick”
6363
{ row: { name: “John”, phone: 3634 },
row: { name: “Sue”, phone: 6343 },
row: { name: “Dick”, phone: 6363 }
}
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XML (Extensible Markup Language)
A W3C standard to complement HTML
Origins: Structured text SGML
• Large-scale electronic publishing
• Data exchange on the web
Motivation:
• HTML describes presentation
• XML describes content
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From HTML to XML
HTML describes the presentation
Database Management Systems, R. Ramakrishnan
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HTML
<h1> Bibliography </h1>
<p> <i> Foundations of Databases </i>
Abiteboul, Hull, Vianu
<br> Addison Wesley, 1995
<p> <i> Data on the Web </i>
Abiteboul, Buneman, Suciu
<br> Morgan Kaufmann, 1999
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XML
<bibliography>
<book> <title> Foundations… </title>
<author> Abiteboul </author>
<author> Hull </author>
<author> Vianu </author>
<publisher> Addison Wesley </publisher>
<year> 1995 </year>
</book>
…
</bibliography>
XML describes the content
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Why are we DB’ers interested?
It’s data. That’s us.
Database issues:
• How are we going to model XML? (graphs).
• How are we going to query XML? (XQuery)
• How are we going to store XML (in a relational
database? object-oriented? native?)
• How are we going to process XML efficiently?
(many interesting research questions!)
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XML Terminology
Tags: book, title, author, …
• start tag: <book>, end tag: </book>
Elements: <book>…<book>,<author>…</author>
• elements can be nested
• empty element: <red></red> (Can be abbrv. <red/>)
XML document: Has a single root element
Well-formed XML document: Has matching tags
Valid XML document: conforms to a schema
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Well-Formed XML
1. Declaration = <? ... ?> .
• Normal declaration is
<? XML VERSION = "1.0" STANDALONE = "yes"
?>
• “Standalone” means that there is no DTD specified.
2. Root tag surrounds the entire balance of the document.
<FOO> is balanced by </FOO>, as in HTML.
3. Any balanced structure of tags OK.
• Option of tags that don’t require balance, like <P> in
HTML.
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XML: An Example
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<BOOKLIST>
<BOOK genre="Science" format="Hardcover">
<AUTHOR>
<FIRSTNAME>Richard</FIRSTNAME><LASTNAME>Feynman</LASTNAME>
</AUTHOR>
<TITLE>The Character of Physical Law</TITLE>
<PUBLISHED>1980</PUBLISHED>
</BOOK>
<BOOK genre="Fiction">
<AUTHOR>
<FIRSTNAME>R.K.</FIRSTNAME><LASTNAME>Narayan</LASTNAME>
</AUTHOR>
<TITLE>Waiting for the Mahatma</TITLE>
<PUBLISHED>1981</PUBLISHED>
</BOOK>
<BOOK genre="Fiction">
<AUTHOR>
<FIRSTNAME>R.K.</FIRSTNAME><LASTNAME>Narayan</LASTNAME>
</AUTHOR>
<TITLE>The English Teacher</TITLE>
<PUBLISHED>1980</PUBLISHED>
</BOOK>
</BOOKLIST>
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XML – Elements
<BOOK genre="Science" format="Hardcover">…</BOOK>
attribute
open tag
element name
attribute value
data
closing tag
Xml is case and space sensitive
Element opening and closing tag names must be identical
Opening tags: “<” + element name + “>”
Closing tags: “</” + element name + “>”
Empty Elements have no data and no closing tag:
• They begin with a “<“ and end with a “/>”
<BOOK/>
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XML – Attributes
<BOOK genre="Science" format="Hardcover">…</BOOK>
attribute
open tag
attribute value
element name
data
closing tag
Attributes provide additional information for element tags.
There can be zero or more attributes in every element; each one
has the the form:
attribute_name=‘attribute_value’
- There is no space between the name and the “=‘”
- Attribute values must be surrounded by “ or ‘ characters
Multiple attributes are separated by white space (one or more
spaces or tabs).
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Elements
The segment of an XML document between an opening
and a corresponding closing tag is called an element.
element
<person>
<name> Malcolm Atchison </name>
<tel> (215) 898 4321 </tel>
<tel> (215) 898 4321 </tel>
<email> mp@dcs.gla.ac.sc </email>
</person>
element, a sub-element
of
Database Management Systems, R. Ramakrishnan
not an element
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XML – Data and Comments
<BOOK genre="Science" format="Hardcover">…</BOOK>
attribute
open tag
attribute value
element name
closing tag
data
Xml data is any information between an opening and closing
tag
Xml data must not contain the ‘<‘ or ‘>’ characters
Comments:
<!- comment ->
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XML text
XML has only one “basic” type -- text.
It is bounded by tags, e.g.
<title> The Big Sleep </title>
<year> 1935 </ year> --- 1935 is still text
XML text is called PCDATA (for parsed
character data). It uses a 16-bit encoding.
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XML – Nesting & Hierarchy
Xml tags can be nested in a tree hierarchy
Xml documents can have only one root tag
Between an opening and closing tag you can insert:
1. Data
2. More Elements
3. A combination of data and elements
<root>
<tag1>
Some Text
<tag2>More</tag2>
</tag1>
</root>
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Representing relational DBs:
Two ways
projects:
title
employees:
name
budget
ssn
Database Management Systems, R. Ramakrishnan
managedBy
age
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Project and Employee relations in XML
Projects and employees are intermixed
<db>
<project>
<employee>
<name> Sandra </name>
<title> Pattern recognition </title>
<ssn> 2234 </ssn>
<budget> 10000 </budget>
<age> 35 </age>
<managedBy> Joe</managedBy>
</employee>
</project>
<project>
<employee>
<title> Auto guided vehicle </title>
<budget> 70000 </budget>
<name> Joe </name>
<managedBy> Sandra </managedBy>
<ssn> 344556 </ssn>
</project>
<age> 34 < /age>
:
</employee>
</db>
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Project and Employee relations in XML (cont’d)
Employees follows projects
<db>
<projects>
<project>
<title> Pattern recognition </title>
<budget> 10000 </budget>
<managedBy>Joe </managedBy>
</project>
<project>
<title>Auto guided vehicles</title>
<budget> 70000 </budget>
<managedBy>Sandra</managedBy>
</project>
:
</projects>
Database Management Systems, R. Ramakrishnan
<employees>
<employee>
<name> Joe </name>
<ssn> 344556 </ssn>
<age> 34 </age>
</employee>
<employee>
<name> Sandra </name>
<ssn> 2234 </ssn>
<age>35 </age>
</employee>
:
<employees>
</db>
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More XML: Oids and References
<person id=“o555”> <name> Jane </name> </person>
<person id=“o456”> <name> Mary </name>
<children idref=“o123 o555”/>
</person>
<person id=“o123” mother=“o456”><name>John</name>
</person>
oids and references in XML are just syntax
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XML Data Model (Graph)
db
#0
book
book
publisher
b1
b2
pub
title
#1
pcdata
mkp
author
#2
pcdata
title
#3
pcdata
pub
author
#5
#4
pcdata
pcdata
Complete... Chamberlin Principles... Bernstein
Database Management Systems, R. Ramakrishnan
author
Newcomer
name
#6
pcdata
state
#7
pcdata
Morgan... CA
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Document Type Descriptors
Sort of like a schema but not really.
<!ELEMENT Book (title, author*) >
<!ELEMENT title #PCDATA>
<!ELEMENT author (name, address,age?)>
<!ATTLIST Book id ID #REQUIRED>
<!ATTLIST Book pub IDREF #IMPLIED>
Inherited from SGML DTD standard
BNF grammar establishing constraints on element
structure and content
Definitions of entities
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DTD – An Example
<?xml version='1.0'?>
<!ELEMENT Basket (Cherry+, (Apple | Orange)*) >
<!ELEMENT Cherry EMPTY>
<!ATTLIST Cherry flavor CDATA #REQUIRED>
<!ELEMENT Apple EMPTY>
<!ATTLIST Apple color CDATA #REQUIRED>
<!ELEMENT Orange EMPTY>
<!ATTLIST Orange location ‘Florida’>
--------------------------------------------------------------------------------
<Basket>
<Cherry flavor=‘good’/>
<Apple color=‘red’/>
<Apple color=‘green’/>
</Basket>
Database Management Systems, R. Ramakrishnan
<Basket>
<Apple/>
<Cherry flavor=‘good’/>
<Orange/>
</Basket>
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DTD - !ELEMENT
<!ELEMENT Basket (Cherry+, (Apple | Orange)*) >
Name
Children
!ELEMENT declares an element name, and
what children elements it should have
Content types:
•
•
•
•
•
Other elements
#PCDATA (parsed character data)
EMPTY (no content)
ANY (no checking inside this structure)
A regular expression
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DTD - !ELEMENT (Contd.)
A regular expression has the following
structure:
• exp1, exp2, exp3, …, expk: A list of regular
expressions
• exp*: An optional expression with zero or more
occurrences
• exp+: An optional expression with one or more
occurrences
• exp1 | exp2 | … | expk: A disjunction of
expressions
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DTD - !ATTLIST
<!ATTLIST Cherry flavor CDATA #REQUIRED>
Element Attribute
Type
Flag
<!ATTLIST Orange location CDATA #REQUIRED
color ‘orange’>
!ATTLIST defines a list of attributes for an
element
Attributes can be of different types, can be
required or not required, and they can have
default values.
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DTD – Well-Formed and Valid
<?xml version='1.0'?>
<!ELEMENT Basket (Cherry+)>
<!ELEMENT Cherry EMPTY>
<!ATTLIST Cherry flavor CDATA #REQUIRED>
--------------------------------------------------------------------------------
Not Well-Formed
Well-Formed but Invalid
<basket>
<Job>
<Cherry flavor=good>
<Location>Home</Location>
</Basket>
</Job>
Well-Formed and Valid
<Basket>
<Cherry flavor=‘good’/>
</Basket>
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Example: An Address Book
<person>
<name> MacNiel, John </name>
<greet> Dr. John MacNiel </greet>
<addr>1234 Huron Street </addr>
<addr> Rome, OH 98765 </addr>
<tel> (321) 786 2543 </tel>
<fax> (321) 786 2543 </fax>
<tel> (321) 786 2543 </tel>
<email> jm@abc.com </email>
</person>
Database Management Systems, R. Ramakrishnan
Exactly one name
At most one greeting
As many address lines
as needed (in order)
Mixed telephones
and faxes
As many
as needed
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Specifying the structure
name
to specify a name element
greet?
to specify an optional
(0 or 1) greet elements
name,greet?
to specify a name followed by
an optional greet
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Specifying the structure (cont)
addr*
to specify 0 or more address lines
tel | fax
a tel or a fax element
(tel | fax)*
0 or more repeats of tel or fax
email*
0 or more email elements
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A DTD for the address book
<!DOCTYPE addressbook [
<!ELEMENT addressbook (person*)>
<!ELEMENT person
(name, greet?, address*, (fax | tel)*, email*)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT greet (#PCDATA)>
<!ELEMENT address (#PCDATA)>
<!ELEMENT tel
(#PCDATA)>
<!ELEMENT fax
(#PCDATA)>
<!ELEMENT email (#PCDATA)>
]>
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DTD for the example relational DB
<!DOCTYPE db [
<!ELEMENT db
<!ELEMENT projects
<!ELEMENT employees
<!ELEMENT project
<!ELEMENT employee
...
]>
Database Management Systems, R. Ramakrishnan
(projects,employees)>
(project*)>
(employee*)>
(title, budget, managedBy)>
(name, ssn, age)>
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Summary of XML regular
expressions
Each element name is a tag.
Its components are the tags that appear nested
within, in the order specified.
A
The tag A occurs
e1,e2
The expression e1 followed by e2
e*
0 or more occurrences of e
e?
Optional -- 0 or 1 occurrences
e+
1 or more occurrences
e1 | e2
either e1 or e2
(e)
grouping
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XML Querying
Path Expressions :
Bib.paper
Bib.book.publisher
Bib.paper.author.lastname
Given an OEM instance, the value of a path
expression p is a set of objects
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Path Expressions
Bib
&o1
Examples:
paper
paper
book
references
&o12
&o24
references
DB =
author
title
&o43
year
&o44
&o29
references
author
http
author
title publisher
title
author
author
author
&o45 &o46
&o52
page
&25
&96
1997
firstname
lastname
&o70
“Serge”
&o47 &o48 &o49 &o50 &o51
firstname
&o71
“Abiteboul”
Bib.paper={&o12,&o29}
Bib.book.publisher={&o51}
Bib.paper.author.lastname={&o71,&206}
Database Management Systems, R. Ramakrishnan
&243
“Victor”
last
lastname
first
&206
“Vianu”
122
133
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XQuery
Emerging standard for querying XML documents. Basic form:
FOR <variables ranging over sets of elements>
WHERE <condition>
RETURN <set of elements>;
Sets of elements described by paths, consisting of:
1. URL, if necessary.
2. Element names forming a path in the semistructured data
graph, e.g., //BAR/NAME =
“start at any BAR node and go to a NAME child.”
3. Ending condition of the form
[<condition about subelements, @attributes, and values>]
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XQuery
Overview:
FOR-LET-WHERE-ORDERBY-RETURN = FLWOR
FOR/LET Clauses
List of tuples
WHERE Clause
List of tuples
ORDERBY/RETURN Clause
Database Management Systems, R. Ramakrishnan
Instance of Xquery data model
45
XQuery
FOR $x in expr -- binds $x to each value in
the list expr
LET $x = expr -- binds $x to the entire list
expr
• Useful for common subexpressions and for
aggregations
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FOR v.s. LET
FOR $x IN document("bib.xml")/bib/book
RETURN <result> $x </result>
LET $x IN document("bib.xml")/bib/book
RETURN <result> $x </result>
Database Management Systems, R. Ramakrishnan
Returns:
<result> <book>...</book></result>
<result> <book>...</book></result>
<result> <book>...</book></result>
...
Returns:
<result> <book>...</book>
<book>...</book>
<book>...</book>
...
</result>
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XQuery
Find all book titles published after 1995:
FOR $x IN document("bib.xml")/bib/book
WHERE $x/year > 1995
RETURN $x/title
Result:
<title> abc </title>
<title> def </title>
<title> ghi </title>
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XQuery
For each author of a book by Morgan
Kaufmann, list all books s/he published:
FOR $a IN distinct(document("bib.xml")
/bib/book[publisher=“Morgan Kaufmann”]/author)
RETURN <result>
$a,
FOR $t IN /bib/book[author=$a]/title
RETURN $t
</result>
distinct = a function that eliminates duplicates
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XQuery
Result:
<result>
<author>Jones</author>
<title> abc </title>
<title> def </title>
</result>
<result>
<author> Smith </author>
<title> ghi </title>
</result>
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XQuery
<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>
count = a (aggregate) function that returns the number of elms
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XQuery
Find books whose price is larger than average:
LET $a=avg(document("bib.xml")/bib/book/price)
FOR $b in document("bib.xml")/bib/book
WHERE $b/price > $a
RETURN $b
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Examples for XQuery queries
FOR $x IN
doc(www.company.com/info.xml)
//employee [employeeSalary gt 70000]/employeeName
RETURN <res> $x/firstName, $x/lastName </res>
FOR $x IN
doc(www.company.com/info.xml)/company/employee
WHERE $x/employeeSalary gt 70000
RETURN <res> $x/employeeName/firstName,
$x/employeeName/lastName </res>
FOR $x IN
doc(www.company.com/info.xml)/company
/project [projectNumber = 5]/projectWorker,
$y IN
doc(www.company.com/info.xml)/company/employee
WHERE $x/hours gt 20.0 AND $y.ssn = $x.ssn
RETURN <res> $x/EmployeeName/firstName,
$y/employeeName/lastName, $x/hours </res>
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