Transcript xml

Introduction to Semistructured
Data and XML
Chapter 27
Database Management Systems, R. Ramakrishnan
1
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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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 }
}
9
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
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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 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> [email protected] </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
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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
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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> [email protected] </email>
</person>
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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
...
]>
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(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 Namespaces


Namespaces are a simple and straightforward way to
distinguish names used in XML documents, no matter where
they come from
The only reason namespaces exist, is to give elements and
attributes programmer-friendly names that will be unique
across the whole Internet
41
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Example
<h:html xmlns:xdc="http://www.xml.com/books"
xmlns:h="http://www.w3.org/HTML/1998/html4">
<h:head><h:title>Book
Review</h:title></h:head>
<h:body>
<xdc:bookreview> <xdc:title>XML: A
Primer</xdc:title>
<h:table> <h:tr align="center">
<h:td>Author</h:td><h:td>Price</h:td>
<h:td>Pages</h:td><h:td>Date</h:td></h:tr>
<h:tr align="left">
<h:td><xdc:author>Simon St.
Laurent</xdc:author></h:td>
<h:td><xdc:price>31.98</xdc:price></h:td>
<h:td><xdc:pages>352</xdc:pages></h:td>
<h:td><xdc:date>1998/01</xdc:date></h:td>
</h:tr> </h:table> </xdc:bookreview>
42
</h:body>
</h:html>
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XML Namespaces



The prefixes are linked to the full names using the attributes
on the top element whose names begin xmlns:
The prefixes are just shorthand placeholders for the full
names
Those full names are URIs, i.e. Web addresses
43
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Extensibility in XML




Anyone can invent new tags and attach a meaning to
those tags
But if every user creates its own XML definition for
describing his data, it is not possible to achieve
interoperability
For example, one may prefer to use the tag name
“POR”, while another prefers using the tag name
“PurchaseOrderReq”
In other words, a tagged document is not very
useful without some kind of agreement on the tags
among inter-operating applications
44
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Many Efforts for Standardized
Tags…
HL7 for healthcare
 RosettaNet for supply chain integration in
Information Technology and Electronic
Components domain
 MPEG7 in multimedia applications
 ebXML for eBusiness
 Common Business Library (CBL) for
electronic catalogs, purchase orders, etc.
…

45
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45
XML Parsers

A parser takes an XML document and makes its
structure and content available to an application
through an API

There are two main Application Programming
Interfaces (APIs) for writing parsers:
• Document Object Model (DOM) and
• Simple API for XML (SAX)

Today, many parsers are both DOM and SAX
compliant
46
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XML DOM Parser
Application Code
Begin parsing
Initialize Parser
In memory
DOM: Perform
Processing
Parsing complete
XML
Parser
XML
Document
A parser validates and makes the data
contained in an XML document available
to the application
47
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XSLT Processor
XSLT Processor
XSL Style
Sheet 1
Parser
XSL Style
Sheet 2
Output from
Style
Sheet 1
Output from
Style
Sheet 2
XML
Document
• Converts an XML document to
another form
• An XSL style sheet is a set of
transformation instructions for
converting a source XML document to
a target document
48
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table.xsl
bar.xsl
art.xsl
49
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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}
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&243
“Victor”
last
lastname
first
&206
“Vianu”
122
133
51
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
53
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>
55
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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transcript.xml
<Transcripts>
<Transcript>
<Student StudId=“111111111” Name=“John Doe” />
<CrsTaken CrsCode=“CS308” Semester=“F1997” Grade=“B” />
<CrsTaken CrsCode=“MAT123” Semester=“F1997” Grade=“B” />
<CrsTaken CrsCode=“EE101” Semester=“F1997” Grade=“A” />
<CrsTaken CrsCode=“CS305” Semester=“F1995” Grade=“A” />
</Transcript>
<Transcript>
<Student StudId=“987654321” Name=“Bart Simpson” />
<CrsTaken CrsCode=“CS305” Semester=“F1995” Grade=“C” />
<CrsTaken CrsCode=“CS308” Semester=“F1994” Grade=“B” />
</Transcript>
… … cont’d … …
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transcript.xml (cont’d)
<Transcript>
<Student StudId=“123454321” Name=“Joe Blow” />
<CrsTaken CrsCode=“CS315” Semester=“S1997” Grade=“A” />
<CrsTaken CrsCode=“CS305” Semester=“S1996” Grade=“A” />
<CrsTaken CrsCode=“MAT123” Semester=“S1996” Grade=“C” />
</Transcript>
<Transcript>
<Student StudId=“023456789” Name=“Homer Simpson” />
<CrsTaken CrsCode=“EE101” Semester=“F1995” Grade=“B” />
<CrsTaken CrsCode=“CS305” Semester=“S1996” Grade=“A” />
</Transcript>
</Transcripts>
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XQuery

Example:
FOR $t IN
document(“http://xyz.edu/transcript.xml”)/Transcript
WHERE $t/CrsTaken/@CrsCode = “MAT123”
RETURN $t/Student

Result:
<Student StudId=“111111111” Name=“John Doe” />
<Student StudId=“123454321” Name=“Joe Blow” />
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XQuery (cont’d)

Previous query doesn’t produce a well-formed
XML document; the following does:
FLWR
<StudentList>
{
inside XML
FOR $t IN document(“transcript.xml”)/Transcript
WHERE $t/CrsTaken/@CrsCode = “MAT123”
RETURN $t/Student
}
</StudentList>

FOR binds $t to Transcript elements one by one, filters
using WHERE, then places Student-children as children of
StudentList using RETURN
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Document Restructuring with XQuery

Reconstruct lists of students taking each class using the
Transcript records:
FOR $c IN distinct(document(“transcript.xml”)/CrsTaken)
RETURN
<ClassRoster CrsCode = {$c/@CrsCode} Semester =
{$c/@Semester}>
{
FOR $t IN document(“transcript.xml”)/Transcript
WHERE $t/CrsTaken/@CrsCode = $c/CrsCode
AND $t/CrsTaken/@Semester = $c/@Semester
RETURN $t/Student
ORDERBY ($t/Student/@StudId)
FLWOR inside
}
RETURN
</ClassRoster>
ORDERBY ($c/@CrsCode)
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66
Document Restructuring (cont’d)

Output elements have the form:
<ClassRoster CrsCode=“CS305” Semester=“F1995” >
<Student StudId=“111111111” Name=“John Doe” />
<Student StudId=“987654321” Name=“Bart Simpson” />
</ClassRoster>

Problem: the above element will be output twice – once
when $c is bound to
<CrsTaken CrsCode=“CS305” Semester=“F1995” Grade=“A” />
and once when it is bound to
Bart Simpson’s
John Doe’s
<CrsTaken CrsCode=“CS305” Semester=“F1995” Grade=“C” />
Note: grades are different – distinct( ) won’t eliminate
transcript records that refer to same class!
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67
Document Restructuring (cont’d)

Solution: instead of
FOR $c IN distinct(document(“transcript.xml”)/CrsTaken)
use
FOR $c IN document(“classes.xml”)//Class
Document
on next
slide
where classes.xml lists course offerings (course
code/semester) explicitly (no need to extract them from
transcript records).
Then $c is bound to each class exactly once, so each class
roster will be output exactly once
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http://xyz.edu/classes.xml
<Classes>
<Class CrsCode=“CS308” Semester=“F1997” >
<CrsName>SE</CrsName> <Instructor>Adrian Jones</Instructor>
</Class>
<Class CrsCode=“EE101” Semester=“F1995” >
<CrsName>Circuits</CrsName> <Instructor>David Jones</Instructor>
</Class>
<Class CrsCode=“CS305” Semester=“F1995” >
<CrsName>Databases</CrsName> <Instructor>Mary Doe</Instructor>
</Class>
<Class CrsCode=“CS315” Semester=“S1997” >
<CrsName>TP</CrsName> <Instructor>John Smyth</Instructor>
</Class>
<Class CrsCode=“MAR123” Semester=“F1997” >
<CrsName>Algebra</CrsName> <Instructor>Ann White</Instructor>
</Class>
</Classes>
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XQuery Semantics
So far the discussion was informal
 XQuery semantics defines what the expected
result of a query is
 Defined analogously to the semantics of SQL

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XQuery Semantics (cont’d)

Step 1: Produce a list of bindings for variables
• The FOR clause binds each variable to an ordered
list of nodes specified by an XQuery expression.
The expression can be:
• An XQuery query
• A function that returns a list of nodes
• End result of a FOR clause:
• Ordered list of tuples of document nodes
• Each tuple is a binding for the variables in the FOR
clause
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XQuery Semantics (cont’d)
Example (bindings):
• Let FOR declare $A and $B
• Bind $A to document nodes {v,w}; $B to {x,y,z}
• Then FOR clause produces the following list of
bindings for $A and $B:
• $A/v, $B/x
• $A/v, $B/y
• $A/v, $B/z
• $A/w, $B/x
• $A/w, $B/y
• $A/w, $B/z
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XQuery Semantics (cont’d)

Step 2: filter the bindings via the WHERE clause
• Use each tuple-binding to substitute its components for
variables; retain those bindings that make WHERE true
• Example: WHERE $A/CrsTaken/@CrsCode =
$B/Class/@CrsCode
• Binding: $A/w, where w = <CrsTaken CrsCode=“CS308” …/>
$B/x, where x = <Class CrsCode=“CS308” … />
• Then w/CrsTaken/@CrsCode = x/Class/@CrsCode, so the
WHERE condition is satisfied & binding retained
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XQuery Semantics (cont’d)

Step 3: Construct result
• For each retained tuple of bindings, instantiate the
RETURN clause
• This creates a fragment of the output document
• Do this for each retained tuple of bindings in sequence
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