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Unit objectives
 Identify the limitations of DTDs
 Describe schema and its components
 Create and modify schema
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 Topic A: XML Schema
 Topic B: Schema description and components
 Topic C: Creating a schema
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 Document type definitions (DTDs) provide a mechanism for modeling XML languages
 Developed to overcome the limitations of DTDs
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 Special syntax
 Can not be processed by a standard XML processor
 Limited ability to describe the data in elements and attributes
 Limited support for namespaces
 New names required for similar elements in different contexts of the same document
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XML Schema recommendations
Schema
 An application of XML
 Defines a model for describing the structure and content of data
 Referred to as XML Schema or XSchema
 Facilitates development of better data descriptions and definitions of shared markup vocabularies
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 Topic A: XML Schema
 Topic B: Schema description and components
 Topic C: Creating a schema
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 Constraint defines what can appear in a given language or document
 Content model constraint
 Defines the elements that can appear
 Datatype constraint
 Describes the units of data that the schema considers valid
 Class of documents - individual schemas
 Instance documents - XML documents that conform to a particular schema
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 The prolog
 First 3 lines of the schema
 First line is the XML declaration (<?xml..?>) - mandatory
 Next 2 lines are comment lines (<!-….-- >)
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 Namespace in schema
 <xs:schema ..> schema element
 Conforms to W3C Schema recommendation
 Namespace declaration
 xmlns:xs=http://www.w3.org/2001/XMLSchema
 “xmlns” - namespace declaration
 “xs” - abbreviation to relate the respective elements or datatypes to the namespace
 http://www.w3.org/2001/XMLSchema - the unique Uniform
Resource Identifier (URI) continued
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 Qualified and unqualified locals
 Local elements are declared in subelements of the schema element, but not in the scope of schema
 Global declared elements are declared in the
<schema> element
 Prefixed element names are qualified names
 Element names without prefixes are unqualified names
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 Complex type
 Determines appearance and content in XML-related instance documents
 Contains attributes and/or subelements
 Simple type
 Contains no attributes or subelements, just character data
 Compositors
 Define groups of element and attributes
 Might be nested inside of one another
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 Sequence compositor
 Elements must appear in the same order as indicated in the schema
 Choice compositor
 Allows only one element to appear in the XML document at that point
 All compositor
 Allows any of the elements to appear in any order
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 The number of times an element may occur in instance document
 Denoted by a plus sign (+) that indicates it must occur at least once
 No limit to the number of times it might occur
 XML Spy substituted attributes:
 minOccur and maxOccur (minimum and maximum number of occurrences)
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 Simple types of elements
 Defined in the W3C Schema Recommendation (XML
Schema)
 Use prefix xs:
 Defined by the schema designer according to specific needs
 Russian Doll or nesting approach - A full element declaration is inserted every time the element is needed
 Flat catalog approach – employs global references
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 Topic A: XML Schema
 Topic B: Schema description and components
 Topic C: Creating a schema
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 You can use XML Spy to create schemas
 You can use XML Spy to convert an existing
DTD to a schema
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 Topic C – Creating a Schema
 C-1: Converting a DTD to Schema
 C-2: Creating XML Schema from scratch
 C-3: Adding Global Components in a Schema
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 Identify the limitations of DTDs
 Described schema and its components
 Learned how to create and modify schema
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