DTD and XML Schema

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XML• Extensible Markup Language

– A standard adopted in 1998 by the W3C (World Wide Web Consortium)

• Optional mechanisms for specifying document structure– DTD: the Document Type Definition Language, part of

the XML standard– XML Schema: a more recent specification built on top of

XML• Query languages for XML

– XPath: lightweight– XSLIT: document transformation language– XQuery: a full-blown language

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Example

Root element

Mandatory statement

XML element

Element name

Element content

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Hierarchical StructurePersonList Student

Title Contents

Person Person

Name: John Doe

Id: 111111111

Address

Number: 123

Street: Main St

Name: Joe Public

Id: 666666666

Address

Number: 666

Street: Hollow Rd

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Document Type Definitions

• A set of rules for structuring an XML document– Specified as part of the document itself, or– Give a URL where its DTD can be found– A document that conforms to its DTD is said valid

• XML does not require a document has a DTD, but it must be well formed

• A grammar that specifies a legal XML document, based on the tags used in the document and their attributes

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Example – DTD

<!DOCTYPE PersonList[<!ELEMENT PersonList (Title, Contents)><!ELEMENT Title EMPTY><!ELEMENT Contents (Person*)><!ELEMENT Person (Name, Id, Address)><!ELEMENT Name (#PCDATA)><!ELEMENT Id (#PCDATA)><!ELEMENT Address (Number, Street)><!ELEMENT Number (#PCDATA)><!ELEMENT Street (#PCDATA)><!ATTLIST PersonList Type CDATA #IMPLIED

Date CDATA #IMPLIED><!ATTLIST Title Value CDATA #REQUIRED>

]>

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DTD Components• Name (e.g., PersonList)

– Must coincide with the tag name of the root element of the document

• One ELEMENT statement for each allowed tag, including the root tag

• For each tag that can have attributes, the ATTLIST statement specifies the allowed attributes and their types

<!DOCTYPE PersonList[<!ELEMENT PersonList (Title, Contents)><!ELEMENT Title EMPTY><!ELEMENT Contents (Person*)><!ELEMENT Person (Name, Id, Address)><!ELEMENT Name (#PCDATA)><!ELEMENT Id (#PCDATA)><!ELEMENT Address (Number, Street)><!ELEMENT Number (#PCDATA)><!ELEMENT Street (#PCDATA)><!ATTLIST PersonList Type CDATA #IMPLIED

Date CDATA #IMPLIED><!ATTLIST Title Value CDATA #REQUIRED>

]>

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Specification

• *: a subelement can appear zero or more times– +: a subelement can appear at least one time

• #PCDATA (parsed character data), CDATA: character strings

• #IMPLIED: an attribute is optional• ?: a subelement is optional

– <!ELEMENT Person (Name, Id, Address?)>• |: alternatives of subelements

– <!ELEMENT Name ((First, Last)|(Last, First))>

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Types for Attributes

• CDATA: character strings

• ID: unique values• IDREF: referential• IDREFS: list of IDREF

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DTD as Data Definition Language

• There are some limitations• Namespaces are not in native design• DTD syntax is quite different from XML• Very limited set of basic types• Limited ways to specify data consistency

constraints– No keys, weak referential integrity, no type references

• No referential integrity for elements• Ordered elements• Global definition of elements

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Why XML Schema?• Use the same syntax as that used for ordinary XML

documents– An alternative to DTD

• Integrated with the namespace mechanism– Different schemas can be imported from different namespaces and

integrated into one schema• Provide a number of built-in types similar to SQL, e.g.,

string, integer, and time• Define complex types from simpler ones• The same element name can be defined as different types

depending on where the element is nested• Support keys and referential integrity constraints• Easy to specify documents where elements are unordered

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Schema and Instance

• Goal: describing XML schema using XML• An XML document D that conforms to a

given schema (which is another XML document) is said to be schema valid– D is called an instance of the schema

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XML Schema and Namespaces• An XML schema document begins with a declaration of the

namespaces to be used• http://www.w3.org/2001/XMLSchema – the namespace

identifying the names of tags and attributes used in a schema (not in the instances)– Describe the structural properties of documents in general, e.g.,

schema, attribute, element, …• http://www.w3.org/2001/XMLSchema-instance – another

namespace used in conjunction with the above one– Identify a small number of special names that are defined in the

XML Schema Specification and are used in the instance documents, e.g., schemaLocation

• The target namespace – identifies the set of names defined by a particular schema document to be used in the instances

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Schema and An Instance Document

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Report Document

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Primitive Types

• DTD has very limited primitive types– CDATA, ID, IDREF, IDREFS

• Many useful primitive types in XML Schema– Decimal, integer, float, Boolean, date, …

• Derive new primitive types from the basic ones– The mechanism is similar to the CREATE

DOMAIN statement in SQL

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Deriving Simple Types

• IDREFS is not one of the primitive types<simpleType name=“myIdrefs”>

<list itemType=“IDREF”/></simpleType>

• Union of multiple typesSuppose local phone numbers are 7 digits long and long

distance numbers are 10 digits long<simpleType name=“phoneNumber”>

<union memberTypes=“phone7digits phone10digits”/></simpleType>

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Deriving Simple Types by Restriction

• Constrain a basic type using one or more constraints from a fixed repertoire defined by the XML Schema specification<simpleType name=“phone7digits”>

<restriction base=“integer”><minInclusive value=“1000000”/><maxInclusive value=“9999999”/>

</restriction></simpleType>

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More Examples

• Phone numbers in XXX-YYYY format<simpleType name=“phone7digitsAndDash”>

<restriction base=“string”><pattern value=“[0-9]{3}-[0-9]{4}”/>

</restriction></simpleType>

• More restrictions on basic string type– <length value=“7”/> – strings of length 7– <minLength value=“7”/> – strings of length >= 7– <maxLength value=“14”/> – strings of length <=14

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Enumeration

• Restrict the domain to a finite set• Can be applied to any base type

<simpleType name=“emergencyNumbers”><restriction base=“integer”><enumeration value=“911”/><enumeration value=“333”/><enumeration value=“5431234”/>

</restriction></simpleType>

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More Examples on Simple Types

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Complex Types

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Basics of Complex Types

• Tag complexType• Tag sequence: a list of elements that must occur

in the given order• Using minOccurs and maxOccurs• Associating attributes with type• A complex type can be associated with an element

<element name=“Student” type=“adm:studentType”/>

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Element without Content

• Just associate attributes with types• Example

<complexType name=“courseTakenType”><attribute name=“CrsCode” type=“adm:courseRef”/><attribute name=“Semester” type=“string”/>

</complexType>

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Compositors• Tags describing how elements can be combined into

groups, e.g., sequence– Required when a tag has complex content– Required even if the type has only one child element!

• Compositor all: allow elements appear in any order<complexType name=“addressType”>

<all><element name=“StreetName” type=“string”/><element name=“StreetNumber” type=“string”/><element name=“city” type=“string”/>

</all></complexType>

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Restrictions on Compositor All• All must appear directly below complexType

<complexType name=“studentType2”><sequence>

<all><element name=“First” type=“string”/><element name=“Last” type=“string”/>

</all><element name=“Address” type=“string”/>

</sequence></complexType>

• No element within all can be repeated<complexType name=“studentType3”>

<all><element name=“First” type=“string”/><element name=“Last” type=“string”/> <element name=“Address” type=“string” minOccurs=“1”

maxOccurs=“unbounded”/></all>

</complexType>

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Compositor Choice<complexType name=“addressType”>

<sequence><choice>

<element name=“POBox” type=“string”/><sequence>

<element name=“Name” type=“string”/><element name=“Number” type=“string”/>

</sequence></choice><element name=“City” type=“string”/>

</sequence></complexType>

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Local Element Names• Two complex types can have elements that share the

same name– Names of students and names of courses– Impossible in DTD, where all element declarations are global

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Anonymous Types

• Useful for types that might not be reused<element name=“Report”>

<complexType><sequence>

<element name=“Students” type=…/><element name=“Classes” type=…/><element name=“Course” type=…/>

</sequence></complexType>

</element>

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Keys

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Foreign Key Constraints

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Summary

• DTD: a set of rules for structuring an XML document

• XML Schema: a more sophisticated tool to specify structures of XML documents– XML Schema is written in XML

• Assignment 3