XML for beginners

2
XML is not…
• A replacement for HTML
• A presentation format
• A programming language
• A network transfer protocol
• A database

3
But then – what is it?
XML is a markup language for
text documents / textual data
XML allows to define languages
(“applications“) to represent text
documents / textual data

4
XML by Example
<article>
<author>Gerhard Weikum</author>
<title>The Web in 10 Years</title>
</article>
• Easy to understand for human users
• Very expressive (semantics along with the data)
• Well structured, easy to read and write from programs

5
XML by Example
<t108>
<x87>Gerhard Weikum</x87>
<g10>The Web in 10 Years</g10>
</t108>
• Hard to understand for human users
• Not expressive (no semantics along with the data)
• Well structured, easy to read and write from programs

6
Possible Advantages of Using XML
• Truly Portable Data
• Easily readable by human users
• Very expressive (semantics near data)
• Very flexible and customizable (no finite tag set)
• Easy to use from programs
• Easy to convert into other representations
(XML transformation languages)
• Many additional standards and tools
• Widely used and supported

7
App. Scenario 1: Content Mgt.
Database with
XML documents
Clients
ConvertersXML2HTML XML2WML XML2PDF

8
App. Scenario 2: Data Exchange
Legacy
System
(e.g., SAP
R/2)
Legacy
System
(e.g.,
Cobol)
XML
Adapter
XML
Adapter
XML
(BMECat, ebXML, RosettaNet, BizTalk, …)
SupplierBuyer
Order

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App. Scenario 3: XML for Metadata
<rdf:RDF
<rdf:Description rdf:about="http://www-dbs/Sch03.pdf">
<dc:title>A Framework for…</dc:title>
<dc:creator>Ralf Schenkel</dc:creator>
<dc:description>While there are...</dc:description>
<dc:publisher>Saarland University</dc:publisher>
<dc:subject>XML Indexing</dc:subject>
<dc:rights>Copyright ...</dc:rights>
<dc:type>Electronic Document</dc:type>
<dc:format>text/pdf</dc:format>
<dc:language>en</dc:language>
</rdf:Description>
</rdf:RDF>

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App. Scenario 4: Document Markup
<article>
<section id=„1“ title=„Intro“>
This article is about <index>XML</index>.
</section>
<section id=„2“ title=„Main Results“>
<name>Weikum</name> <cite idref=„Weik01“/> shows
the following theorem (see Section <ref idref=„1“/>)
<theorem id=„theo:1“ source=„Weik01“>
For any XML document x, ...
</theorem>
</section>
<literature>
<cite id=„Weik01“><author>Weikum</author></cite>
</literature>
</article>

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App. Scenario 4: Document Markup
• Document Markup adds structural and semantic
information to documents, e.g.
– Sections, Subsections, Theorems, …
– Cross References
– Literature Citations
– Index Entries
– Named Entities

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XML
Part 2 – Basic XML Concepts

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XML Documents
What‘s in an XML document?
• Elements
• Attributes
• plus some other details

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A Simple XML Document
<article>
<title>The Web in Ten Years</title>
<text>
<abstract>In order to evolve...</abstract>
<section number=“1” title=“Introduction”>
The <index>Web</index> provides the universal...
</section>
</text>
</article>

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<article>
<text>
</section>
</text>
</article>
Freely definable tags

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Element
Content of
the Element
(Subelements
and/or Text)
<article>
<text>
</section>
</text>
</article>
End Tag
Start Tag

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<article>
<text>
</section>
</text>
</article>
Attributes with
name and value

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Elements in XML Documents
• (Freely definable) tags: article, title, author
– with start tag: <article> etc.
– and end tag: </article> etc.
• Elements: <article> ... </article>
• Elements have a name (article) and a content (...)
• Elements may be nested.
• Elements may be empty: <this_is_empty/>
• Element content are strings with special characters, and/or nested
elements (mixed content if both).
• Each XML document has exactly one root element and forms a
tree.
• Elements with a common parent are ordered.

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Elements vs. Attributes
Elements may have attributes (in the start tag) that have a name
and
a value, e.g. <section number=“1“>.
What is the difference between elements and attributes?
• Only one attribute with a given name per element
• Attributes have no structure, simply strings (while elements can
have subelements)
As a rule of thumb:
• Content into elements
• Metadata into attributes
Example:
<person born=“1912-06-23“ died=“1954-06-07“>
Alan Turing</person> proved that…

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XML Documents as Ordered Trees
article
author title text
sectionabstract
The index
Web
provides …
title=“…“
number=“1“
In order …
Gerhard
Weikum
The Web
in 10 years

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Well-Formed XML Documents
A well-formed document must adher to, among others, the
following rules:
• Every start tag has a matching end tag.
• Elements may nest, but must not overlap.
• There must be exactly one root element.
• Attribute values must be quoted.
• An element may not have two attributes with the same
name.
• Comments and processing instructions may not appear
inside tags.

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Namespace
<dbs:book xmlns:dbs=“http://www-dbs/dbs“>
Unique URI to identify
the namespace
Signal that namespace
definition happens
Prefix as abbrevation
of URI

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Namespace Example
<dbs:book xmlns:dbs=“http://www-dbs/dbs“>
<dbs:description> ... </dbs:description>
<dbs:text>
<dbs:formula>
<mathml:math
xmlns:mathml=“http://www.w3.org/1998/Math/MathML“>
...
</mathml:math>
</dbs:formula>
</dbs:text>
</dbs:book>

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Default Namespace
• Default namespace may be set for an element and its
content (but not its attributes):
<book xmlns=“http://www-dbs/dbs“>
<description>...</description>
<book>
• Can be overridden in the elements by specifying the
namespace there (using prefix or default namespace)

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XML
Part 3 – Defining XML Data Formats

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3.1 Document Type Definitions
Sometimes XML is too flexible:
• For exchanging data, the format (i.e., elements,
attributes and their semantics) must be fixed
⇒Document Type Definitions (DTD) for establishing the
vocabulary for one XML application (in some sense
comparable to schemas in databases)
A document is valid with respect to a DTD if it conforms
to the rules specified in that DTD.
Most XML parsers can be configured to validate.

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DTD Example: Elements
<!ELEMENT article (title,author+,text)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT text (abstract,section*,literature?)>
<!ELEMENT abstract (#PCDATA)>
<!ELEMENT section (#PCDATA|index)+>
<!ELEMENT literature (#PCDATA)>
<!ELEMENT index (#PCDATA)>
Content of the title element
is parsed character data
Content of the article element is a title element,
followed by one or more author elements,
followed by a text element
Content of the text element may
contain zero or more section
elements in this position

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Element Declarations in DTDs
One element declaration for each element type:
<!ELEMENT element_name content_specification>
where content_specification can be
• (#PCDATA) parsed character data
• (child) one child element
• (c1,…,cn) a sequence of child elements c1…cn
• (c1|…|cn) one of the elements c1…cn
For each component c, possible counts can be specified:
– c exactly one such element
– c+ one or more
– c* zero or more
– c? zero or one
Plus arbitrary combinations using parenthesis:
<!ELEMENT f ((a|b)*,c+,(d|e))*>

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More on Element Declarations
• Elements with mixed content:
<!ELEMENT text (#PCDATA|index|cite|glossary)*>
• Elements with empty content:
<!ELEMENT image EMPTY>
• Elements with arbitrary content (this is nothing for
production-level DTDs):
<!ELEMENT thesis ANY>

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Attribute Declarations in DTDs
Attributes are declared per element:
<!ATTLIST section number CDATA #REQUIRED
title CDATA #REQUIRED>
declares two required attributes for element section.
element name
attribute name
attribute type
attribute default

33
Attribute Declarations in DTDs
Attributes are declared per element:
<!ATTLIST section number CDATA #REQUIRED
title CDATA #REQUIRED>
declares two required attributes for element section.
Possible attribute defaults:
• #REQUIRED is required in each element instance
• #IMPLIED is optional
• #FIXED default always has this default value
• default has this default value if the attribute is
omitted from the element instance

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Attribute Types in DTDs
• CDATA string data
• (A1|…|An) enumeration of all possible values of the
attribute (each is XML name)
• ID unique XML name to identify the element
• IDREF refers to ID attribute of some other element
(„intra-document link“)
• IDREFS list of IDREF, separated by white space
• plus some more

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Attribute Examples
<ATTLIST publication type (journal|inproceedings) #REQUIRED
pubid ID #REQUIRED>
<ATTLIST cite cid IDREF #REQUIRED>
<ATTLIST citation ref IDREF #IMPLIED
cid ID #REQUIRED>
<publications>
<publication type=“journal“ pubid=“Weikum01“>
<text>In the Web of 2010, XML <cite cid=„12“/>...</text>
<citation cid=„12“ ref=„XML98“/>
<citation cid=„15“>...</citation>
</publication>
<publication type=“inproceedings“ pubid=“XML98“>
<text>XML, the extended Markup Language, ...</text>
</publication>
</publications>

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Attribute Examples
<ATTLIST publication type (journal|inproceedings) #REQUIRED
pubid ID #REQUIRED>
<ATTLIST cite cid IDREF #REQUIRED>
<ATTLIST citation ref IDREF #IMPLIED
cid ID #REQUIRED>
<publications>
<publication type=“journal“ pubid=“Weikum01“>
<text>In the Web of 2010, XML <cite cid=„12“/>...</text>
<citation cid=„12“ ref=„XML98“/>
<citation cid=„15“>...</citation>
</publication>
<publication type=“inproceedings“ pubid=“XML98“>
<text>XML, the extended Markup Language, ...</text>
</publication>
</publications>

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Linking DTD and XML Docs
• Document Type Declaration in the XML document:
<!DOCTYPE article SYSTEM “http://www-dbs/article.dtd“>
keywords Root element URI for the DTD

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Linking DTD and XML Docs
• Internal DTD:
<?xml version=“1.0“?>
<!DOCTYPE article [
<!ELEMENT article (title,author+,text)>
...
<!ELEMENT index (#PCDATA)>
]>
<article>
...
</article>
• Both ways can be mixed, internal DTD overwrites
external entity information:
<!DOCTYPE article SYSTEM „article.dtd“ [
<!ENTITY % pub_content (title+,author*,text)
]>

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Flaws of DTDs
• No support for basic data types like integers, doubles,
dates, times, …
• No structured, self-definable data types
• No type derivation
• id/idref links are quite loose (target is not specified)

40
XML
Part 4 – Querying XML Data
4.1 XPath
4.2 XQuery

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Querying XML with XPath and XQuery
XPath and XQuery are query languages for XML data, both
standardized by the W3C and supported by various database products.
Their search capabilities include
• logical conditions over element and attribute content
• regular expressions for pattern matching of element names
along paths or subtrees within XML data
+ joins, grouping, aggregation, transformation, etc. (XQuery only)
In contrast to database query languages like SQL an XML query
does not necessarily (need to) know a fixed structural schema
for the underlying data.
A query result is a set of qualifying nodes, paths, subtrees,
or subgraphs from the underyling data graph,
or a set of XML documents constructed from this raw result.

42
4.1 XPath
• XPath is a simple language to identify parts of the XML
document (for further processing)
• XPath operates on the tree representation of the
document
• Result of an XPath expression is a set of elements or
attributes

46
XPath by Example
/literature/book/author retrieves all book authors:
starting with the root, traverses the tree, matches element
names literature, book, author, and returns elements
<author>Suciu, Dan</author>,
<author>Abiteboul, Serge</author>, ...,
<author><firstname>Jeff</firstname>
<lastname>Ullman</lastname></author>
/literature/*/author authors of books, articles, essays, etc.
/literature//author authors that are descendants of literature
/literature//@year value of the year attribute of descendants of literature
/literature//author[firstname] authors that have a subelement firstname
/literature/(book|article)/author authors of books or articles
/literature/book[price < „50“]
/literature/book[author//country = „Germany“]
low priced books
books with German author

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4.2 Core Concepts of XQuery
XQuery is an extremely powerful query language for XML data.
A query has the form of a so-called FLWR expression:
FOR $var1 IN expr1, $var2 IN expr2, ...
LET $var3 := expr3, $var4 := expr4, ...
WHERE condition
RETURN result-doc-construction
The FOR clause evaluates expressions (which may be XPath-style
path expressions) and binds the resulting elements to variables.
For a given binding each variable denotes exactly one element.
The LET clause binds entire sequences of elements to variables.
The WHERE clause evaluates a logical condition with each of
the possible variable bindings and selects those bindings that
satisfy the condition.
The RETURN clause constructs, from each of the variable bindings,
an XML result tree. This may involve grouping and aggregation
and even complete subqueries.

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