XML

1
XML
2
XML

• eXtensible Markup Language
• XML 1.0 a recommendation from W3C, 1998
• Roots SGML (a very nasty language).
• After the roots a format for sharing data

3
Why XML is of Interest to Us

• XML is just syntax for data
• Note we have no syntax for relational data
• But XML is not relational semistructured
• This is exciting because
• Can translate any data to XML
• Can ship XML over the Web (HTTP)
• Can input XML into any application
• Thus data sharing and exchange on the Web

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XML Data Sharing and Exchange
application
application
object-relational
Integrate
XML Data
WEB (HTTP)
Transform
Warehouse
application
relational data
legacy data
Specific data management tasks
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From HTML to XML
HTML describes the presentation
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HTML

• Bibliography
• Foundations of Databases
• Abiteboul, Hull, Vianu
• 
  Addison Wesley, 1995
• Data on the Web
• Abiteoul, Buneman, Suciu
• 
  Morgan Kaufmann, 1999

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XML

• Foundations
• Abiteboul
• Hull
• Vianu
• Addison Wesley
  
• 1995

XML describes the content
8
Web Services

• A new paradigm for creating distributed
  applications?
• Systems communicate via messages, contracts.
• Example order processing system.
• MS .NET, J2EE some of the platforms
• XML a part of the story the data format.

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XML Terminology

• tags book, title, author,
• start tag , end tag
• elements ,
• elements are nested
• empty element abbrv.
• an XML document single root element

well formed XML document if it has matching tags
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More XML Attributes

• Foundations of Databases
• Abiteboul
• 1995

attributes are alternative ways to represent data
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More XML Oids and References

• Jane
• Mary
• idrefo123 o555/
• John

oids and references in XML are just syntax
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XML Semantics a Tree !
data
Mary
Maple
345 Seattle
John
Thailand
23456
person
person
id
address
name
address
name
phone
o555
street
no
city
Mary
Thai
John
23456
Maple
345
Seattle
Order matters !!!
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XML Data

• XML is self-describing
• Schema elements become part of the data
• Reational schema persons(name,phone)
• In XML , , are part of the
  data, and are repeated many times
• Consequence XML is much more flexible
• XML semistructured data

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Relational Data as XML
person
XML
person
row
row
row
phone
name
name
name
phone
phone
John
3634
Sue
Dick
6343
6363

• John
• 3634
• Sue
• 6343
• Dick
• 6363

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XML is Semi-structured Data

• Missing attributes
• Could represent ina table with nulls

John
1234
Joe
? no phone !
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XML is Semi-structured Data

• Repeated attributes
• Impossible in tables

Mary
2345
3456
? two phones !
???
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XML is Semi-structured Data

• Attributes with different types in different
  objects
• Nested collections (no 1NF)
• Heterogeneous collections
• contains both s and s

John
Smith

1234
? structured name !
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Document Type DefinitionsDTD

• part of the original XML specification
• an XML document may have a DTD
• XML document
• well-formed if tags are correctly closed
• Valid if it has a DTD and conforms to it
• validation is useful in data exchange

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Very Simple DTD
((personproduct)) name, office, phone?) (PCDATA)
(PCDATA) description?)

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Very Simple DTD
Example of valid XML document
123456789
John
B432
1234
987654321
Jim
B123
... ...
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DTD The Content Model

• Content model
• Complex a regular expression over other
  elements
• Text-only PCDATA
• Empty EMPTY
• Any ANY
• Mixed content (PCDATA A B C)

contentmodel
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DTD Regular Expressions
DTD
XML
sequence
(firstName, lastName))
. . . . .
. . . . .
optional
. . . . .
. . . . . . . . .
. . . . . . .
. . . . .
Kleene star
alternation
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Querying XML Data

• XPath simple navigation through the tree
• XQuery the SQL of XML
• XSLT recursive traversal

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Sample Data for Queries

• Addison-Wesley
  Serge
  Abiteboul
  Rick
  Hull
  Victor
  Vianu Foundations
  of Databases 1995
  
  Freeman
  Jeffrey D. Ullman
  Principles of Database and Knowledge
  Base Systems 1998
  

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Data Model for XPath
The root
The root element
book
book
publisher
author
. . . .
Addison-Wesley
Serge Abiteboul
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XPath Simple Expressions

• Result 1995
• 1998
• Result empty (there were no papers)

/bib/book/year
/bib/paper/year
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XPath Restricted Kleene Closure

• Result Serge Abiteboul
• Rick
  
• Hull
  
• Victor Vianu
• Jeffrey D. Ullman
  
• Result Rick

//author
/bib//first-name
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Xpath Text Nodes
/bib/book/author/text()

• Result Serge Abiteboul
• Jeffrey D. Ullman
• Rick Hull doesnt appear because he has
  firstname, lastname
• Functions in XPath
• text() matches the text value
• node() matches any node ( or _at_ or text())
• name() returns the name of the current tag

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Xpath Wildcard

• Result Rick
• Hull
• Matches any element

//author/
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Xpath Attribute Nodes
/bib/book/_at_price

• Result 55
• _at_price means that price is has to be an attribute

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Xpath Predicates

• Result Rick

/bib/book/authorfirstname
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Xpath More Predicates

• Result

/bib/book/authorfirstnameaddress//zipcity/
lastname
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Xpath More Predicates
/bib/book_at_price /bib/bookauthor/_at_age /bib/bookauthor/text()
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Xpath Summary

• bib matches a bib element
• matches any element
• / matches the root element
• /bib matches a bib element under root
• bib/paper matches a paper in bib
• bib//paper matches a paper in bib, at any depth
• //paper matches a paper at any depth
• paperbook matches a paper or a book
• _at_price matches a price attribute
• bib/book/_at_price matches price attribute in book,
  in bib
• bib/book/_at_price

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Comments on XPath?

• Whats good about it?
• What cant it do that you want it to do?
• How does it compare, say, to SQL?

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XQuery

• Based on Quilt, which is based on XML-QL
• Uses XPath to express more complex queries

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FLWR (Flower) Expressions

• FOR ...
• LET...
• WHERE...
• RETURN...

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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 abc def
ghi
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XQuery

• Find book titles by the coauthors of Database
  Theory

FOR x IN bib/booktitle/text() Database
Theory/author y IN bib/bookauthor/tex
t() x/text()/title RETURN
y/text()
Result abc
def ghi
The answer willcontain duplicates !
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XQuery

• Same as before, but eliminate duplicates

FOR x IN bib/booktitle/text() Database
Theory/author y IN distinct(bib/booka
uthor/text() x/text()/title) RETURN
y/text()
Result abc
def ghi
distinct a function that eliminates duplicates
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XQuery Nesting

• For each author of a book by Morgan Kaufmann,
  list all books she published

FOR a IN distinct(document("bib.xml")
/bib/bookpublisherMorgan
Kaufmann/author) RETURN
a, FOR t IN
/bib/bookauthora/title
RETURN t

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XQuery

• Jones
• abc
• def
• Smith
• ghi

Result
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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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XQuery
FOR p IN
distinct(document("bib.xml")//publisher)
LET b document("bib.xml")/bookpublisher
p WHERE count(b) 100 RETURN
p
count a (aggregate) function that returns the
number of elms
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XQuery

• Find books whose price is larger than average

LET aavg(document("bib.xml")/bib/book/price) FOR
b in document("bib.xml")/bib/book WHERE
b/price a RETURN b
Lets try to write this in SQL
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XQuery

• Summary
• FOR-LET-WHERE-RETURN FLWR

FOR/LET Clauses
List of tuples
WHERE Clause
List of tuples
RETURN Clause
Instance of Xquery data model
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FOR v.s. LET

• FOR
• Binds node variables ? iteration
• LET
• Binds collection variables ? one value

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FOR v.s. LET
Returns ...
...
... ...
FOR x IN document("bib.xml")/bib/book RETURN
x
LET x IN document("bib.xml")/bib/book RETURN
x
Returns ...
...
... ...
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Collections in XQuery

• Ordered and unordered collections
• /bib/book/author an ordered collection
• Distinct(/bib/book/author) an unordered
  collection
• LET a /bib/book ? a is a collection
• b/author ? a collection (several authors...)

Returns ...
...
...
...
RETURN b/author