New Perspectives on XML, 2nd Edition

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TUTORIAL 4

• WORKING WITH SCHEMAS
• Section 4.1 (p. 145)

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SCHEMAS

• A schema is an XML document that defines the
  content and structure of one or more XML
  documents.
• The XML document containing the content is called
  the instance document.

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COMPARING SCHEMAS AND DTDS

• This figure compares schemas and DTDs

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SCHEMA VOCABULARIES

• There is no single schema form. Several schema
  vocabularies have been developed in the XML
  language.
• Support for a particular schema depends on the
  XML parser being used for validation.

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SCHEMA VOCABULARIES

• This figure shows a few schema vocabularies

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STARTING A SCHEMA FILE

• A schema is always placed in a separate XML
  document that is referenced by the instance
  document.

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ELEMENTS AND ATTRIBUTES OF THE PATIENTS DOCUMENT

• This figure shows the elements and attributes of
  the patients.xml document

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SCHEMA TYPES

• XML Schema recognize two categories of element
  types complex and simple.
• A complex type element has one or more
  attributes, or is the parent to one or more child
  elements.
• A simple type element contains only character
  data and has no attributes.

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SCHEMA TYPES

• This figure shows types of elements

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SIMPLE TYPE ELEMENTS

• Use the following syntax to declare a simple type
  element in XML Schema
• ltelement namename type type/gt
• Here, name is the name of the element in the
  instance document and type is the data type of
  the element.
• If a namespace prefix is used with the XML Schema
  namespace, any XML Schema tags must be qualified
  with the namespace prefix.

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UNDERSTANDING DATA TYPES

• XML Schema supports two data types built-in and
  user-derived.
• A built-in data type is part of the XML Schema
  specifications and is available to all XML Schema
  authors.
• A user-derived data type is created by the XML
  Schema author for specific data values in the
  instance document.

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DECLARING AN ATTRIBUTE

• An attribute is another example of a simple type.
  The syntax to define an attribute is
• ltxsattribute name"name" type"type
  default"default fixed"fixed" /gt
• Where name is the name of the attribute, type is
  the data type, default is the attributes default
  value, and fixed is a fixed value for the
  attribute.

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ASSOCIATING ATTRIBUTES AND ELEMENTS

• The basic structure for defining a complex type
  element with XML Schema is
• ltxselement name"name"gt
• ltxscomplexTypegt
• declarations
• lt/xscomplexTypegt
• lt/xselementgt
• Where name is the name of the element and
  declarations is schema commands specific to the
  type of complex element being defined.

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ASSOCIATING ATTRIBUTES AND ELEMENTS

• Four complex type elements that usually appear in
  an instance document are the following
• The element is an empty element and contains only
  attributes.
• The element contains textual content and
  attributes but no child elements.
• The element contains child elements but not
  attributes.
• The element contains both child elements and
  attributes.

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EMPTY ELEMENTS AND ATTRIBUTES

• The code to declare the attributes of an empty
  element is
• ltxselement name"name"gt
• ltxscomplexTypegt
• attributes
• lt/xscomplexTypegt
• lt/xselementgt
• Where attributes is the set of declarations that
  define the attributes associated with the
  element. For example, the empty element

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SIMPLE CONTENT AND ATTRIBUTES

• If an element is not empty and contains textual
  content (but no child elements), the structure of
  the complex type element is slightly different.
• ltxselement name"name"gt
• ltxscomplexTypegt
• ltxssimpleContentgt
• ltxsextension base"type"gt
• attributes
• lt/xsextensiongt
• lt/xssimpleContentgt
• lt/xscomplexTypegt
• lt/xselementgt

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SPECIFYING THE USE OF AN ATTRIBUTE

• An attribute may or may not be required with a
  particular element. To indicate whether an
  attribute is required, you add the use attribute
  to the element declaration or reference. The use
  attribute has the following values
• requiredThe attribute must always appear with
  the element
• optionalThe use of the attribute is optional
  with the element
• prohibitedThe attribute cannot be used with the
  element

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REFERENCING AN ELEMENT OR ATTRIBUTE

• XML Schema allows for a great deal of flexibility
  in designing complex types. Rather than nesting
  the attribute declaration within the element, you
  can create a reference to it. The code to create
  a reference to an element or attribute
  declaration is
• ltxselement ref"elemName" /gt
• ltxsattribute ref"attName" /gt
• Where elemName is the name used in an element
  declaration and attName is the name used in an
  attribute declaration

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WORKING WITH CHILD ELEMENTS

• Another kind of complex type element contains
  child elements, but no attributes. To define
  these child elements, use the code structure
• ltxselement name"name"gt
• ltxscomplexTypegt
• ltxscompositorgt
• elements
• lt/xscompositorgt
• lt/xscomplexTypegt
• lt/xselementgt
• Where elements is the list of simple type element
  declarations for each child element, and
  compositor defines how the child elements are
  organized.

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USING COMPOSITORS

• XML Schema supports the following compositors
• sequence defines a specific order for the child
  elements
• choice allows any one of the child elements to
  appear in the instance document
• all allows any of the child elements to appear in
  any order in the instance document however, they
  must appear either only once or not all.

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WORKING WITH CHILD ELEMENTS AND ATTRIBUTES

• The code for a complex type element that contains
  both attributes and child elements is
• ltxselement name"name"gt
• ltxscomplexTypegt
• ltxscompositorgt
• elements
• lt/xscompositorgt
• lt/xscomplexTypegt
• attributes
• lt/xselementgt

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SPECIFYING MIXED CONTENT

• When the mixed attribute is set to the value
  true, XML Schema assumes that the element
  contains both text and child elements. The
  structure of the child elements can then be
  defined with the conventional method. For
  example, the XML content
• ltSummarygt
• Patient ltNamegtCynthia Davislt/Namegt was enrolled
  in
• the ltStudygtTamoxifen Studylt/Studygt on 8/15/2003.
• lt/Summarygt
• can be declared in the schema file using the
  following complex type
• ltelement name"Summary"gt
• ltcomplexType mixed"true"gt
• ltsequencegt
• ltelement name"Name" type"string"/gt
• ltelement name"Study" type"string"/gt
• lt/sequencegt
• lt/complexTypegt
• lt/elementgt

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APPLYING A SCHEMA

• To attach a schema to the document, you must do
  the following
• Declare a namespace for XML Schema in the
  instance document.
• Indicate the location of the schema file.
• To declare the XML Schema namespace in the
  instance document, you add the following
  attribute to the documents root element
• xmlnsxsi"http//www.w3.org/2001/XMLSchema-instan
  ce"

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APPLYING A SCHEMA

• If there is no namespace for the contents of the
  instance document, add the following attribute to
  the root element
• xsinoNamespaceSchemaLocation"schema"