of 33 lecture 3: xml and xml schema. of 33 xml, rdf, rdf schema overview xml – simple introduction...
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lecture 3: xml and xml schema
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XML, RDF, RDF Schemaoverview
XML – simple introduction and XML SchemaRDF – basics, languageRDF Schema – basics, languageSemantics of RDF and RDF SchemaSPARQL – query language for RDF
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XMLintroduction
XML – Extensible Markup Language designed to describe structured documents users may create their own tags (they can
create their own specific languages) tags have no semantics indicting how to
present documents through a Web browser
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XMLexample
<? xml version=“1.0” encoding=“UTF-8” ?><book>
<title>Semantic Web is Cool</title><author>John Smith</author><publisher>Springer</publisher><year>1993</year><ISBN>0387976892</ISBN>
</book>
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XML prolog of a document
the prolog: an XML declaration an optional reference to external structuring
documents
<?xml version="1.0" encoding="UTF-8"?>
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XML elements
“things” the XML document talks about books, authors, publishers, …
each element contains three parts an opening tag the content a closing tag
<author>John Smith</author>
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XMLelements (2)
tag names can be chosen almost freely the first character must be a letter, an underscore, or a colonno name may begin with the string “xml” in any combination of cases (“Xml”, “xML”)
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XMLcontent of elements
content may be text, or other elements, or nothing
<author><name>John Smith</name><phone> +1 − 780 − 492 5507
</phone></author>
if no content
<author/> for <author></author>
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XML attributes
an empty element is not necessarily meaningless it may have some properties in terms of attributes
an attribute is a name-value pair inside the opening tag of an element
<author name=”John Smith" phone="+1 − 780 − 492 5507"/>
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XML other components
comments<!-- This is a comment -->
processing instructions (define procedural attachments)
<?stylesheet type="text/css” href="mystyle.css"?>
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XMLwell-formed documents
syntactically correct documents – ones that obey some syntactic rules: there is only one outermost element (called root
element) each element has an opening and a
corresponding closing tag tags may not overlap
<author><name>Lee Hong</author></name> attributes have unique names names of elements and tags must be permissible
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XMLtree model of XML documents
<email> <head>
<from name=”John Smith" address=”[email protected]"/>
<to name=”Jenny Doe"address=”[email protected]"/>
<subject>How are you?</subject> </head> <body>
Hi, it was nice … </body></email>
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XMLtree model of XML documents
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XML structure of documents
definition of all element and attribute names that may be useddefinition of structure what values an attribute may take which elements may or must occur within other
elements, etc.
if such structuring information exists, the document can be validated
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an XML document is valid if it is well-formed respects the structuring information it uses
there are two ways of defining the structure of XML documents:
DTDs (the older and more restricted way) XML Schema (offers extended possibilities)
XML structure of documents (2)
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<author><name>John Smith</name><phone> +1 − 780 − 492 5507 </phone>
</author>
DTD for above element (and all author elements):<!ELEMENT author (name,phone)><!ELEMENT name (#PCDATA)><!ELEMENT phone (#PCDATA)>
XML structure of documents: DTD
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XMLDTD: its meaning
the element types author, name, and phone may be used in the document
an author element contains a name element and a phone element, in that order (sequence)
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XMLDTD: its meaning
a name element and a phone element may have any content
in DTDs, #PCDATA is the only atomic type for elements
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XML Schema
richer language for defining the structure of XML documents
its syntax is based on XML itself
sophisticated set of data types, compared to DTDs (which only supports strings)
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XML Schema (2)
it is like an element with an opening tag like
<xsd:schema xmlns:xsd=“http://www.w3.org/2001
XMLSchema” version=“1.0”>...</xsd:schema>
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XML Schemaelement types
<element name=”…"/>
type=“…”
minOccurs=“x ” (x may be any natural number)maxOccurs=“x ” (any number of unbounded)
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XML Schemaelement types - examples
<element name="email"/>
<element name="head" minOccurs="1" maxOccurs="1"/>
<element name="to" minOccurs="1"/>
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XML Schemaattribute types
<attribute name=”…"/>
type=“…”
use= “x ” (x may be optional or required or prohibited)
default value
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XML Schemaattribute types - examples
<attribute name="id" type="ID“ use="required"/>
< attribute name="speaks" type="Language" use="default" value="en"/>
existence: use="x", where x may be optional or required
default value: use="x" value="...", where x may be default or fixed
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XML Schemadata types
built-in data types numerical data: integer, Short, … string: string, ID, IDREF, CDATA, … date and time: time, Month, …
user-defined data types simple data types, which cannot use elements
or attributes complex data types, which can use these
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XML Schemadata types (2)
complex data types are defined from already existing data types by defining some attributes (if any) and using: sequence, a sequence of existing data type
elements (order is important) all, a collection of elements that must appear
(order is not important) choice, a collection of elements, of which one
will be chosen
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XML Schemadata type extension
already existing data types can be extended by new elements or attributes<complexType name="extendedLecturerType"> <extension base="lecturerType">
<sequence><element name="email" type="string"
minOccurs="0" maxOccurs="1"/></sequence><attribute name="rank" type="string"
use="required"/> </extension></complexType>
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XML Schemadata type restriction
an existing data type may be restricted by adding constraints on certain values (it is not the opposite process from extension)
the following hierarchical relationship still holds: instances of the restricted type are also instances of the
original type (they satisfy at least the constraints of the original type and some new ones)
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XML Schemadata type restriction – example
<complexType name="restrictedLecturerType"><restriction base="lecturerType">
<sequence><element name="firstname"
type="string"minOccurs="1" maxOccurs="2"/>
</sequence><attribute name="title" type="string"
use="required"/></restriction>
</complexType>
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XML Schemarestriction of simple data types
<simpleType name="dayOfMonth"><restriction base="integer">
<minInclusive value="1"/><maxInclusive value="31"/>
</restriction></simpleType>
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XML Schemanamespaces
a single XML document may use more than one DTD or schema in order to avoid clashes a different prefix for each DTD or schema can/should be used
prefix:name
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XML Schemanamespaces
namespaces are declared within an element and can be used in that element and any of its children (elements and attributes) a namespace declaration has the form:
xmlns:prefix="location"location is the address of the DTD or schema
if a prefix is not specified: xmlns="location" then the location is used by default
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XML Schemanamespaces – example
<… xmlns="http://www.ua.ca/basic.xsd"xmlns:staff="http://www.ua.ca/staff.xsd">
<staff:faculty staff:title=“professor"staff:name="John Smith"staff:department=”ECE"/>
<academicStaff title="lecturer"name=”Jenny Doe"school="Information
Technology"/></…>