XML Schemas: Understanding XSD for Structuring XML Data, Slides of Computer Science

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XML Schemas

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XML Schemas

• “Schemas” is a general term--DTDs are a form of XML schemas - According to the dictionary, a schema is “a structured framework or plan”
• When we say “XML Schemas,” we usually mean the W3C XML Schema Language - This is also known as “XML Schema Definition” language, or XSD - I’ll use “XSD” frequently, because it’s short
• DTDs, XML Schemas, and RELAX NG are all XML schema languages

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Why not XML schemas?

• DTDs have been around longer than XSD
  • Therefore they are more widely used
  • Also, more tools support them
• XSD is very verbose, even by XML standards
• More advanced XML Schema instructions can be non-intuitive and confusing
• Nevertheless, XSD is not likely to go away quickly

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Referring to a schema

• To refer to a DTD in an XML document, the reference goes before the root element: - ...
• To refer to an XML Schema in an XML document, the reference goes in the root element: - <rootElement xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" (The XML Schema Instance reference is required) xsi:noNamespaceSchemaLocation=" url .xsd"> (This is where your XML Schema definition can be found) ...

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• The element may have attributes:
  • xmlns:xs="http://www.w3.org/2001/XMLSche ma" - This is necessary to specify where all our XSD tags are defined
  • elementFormDefault="qualified"
    • This means that all XML elements must be qualified (use a namespace)
    • It is highly desirable to qualify all elements, or problems will arise when another schema is added

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“Simple” and “complex”

elements

• A “simple” element is one that contains text and nothing else - A simple element cannot have attributes - A simple element cannot contain other elements - A simple element cannot be empty - However, the text can be of many different types, and may have various restrictions applied to it
• If an element isn’t simple, it’s “complex”
  • A complex element may have attributes
  • A complex element may be empty, or it may Docsity.com

Defining an attribute

• Attributes themselves are always declared as simple types
• An attribute is defined as <xs:attribute name=" name " type=" type " /> where: - name and type are the same as for xs:element
• Other attributes a simple element may have:
  • default=" default value " if no other value is specified Docsity.com

Restrictions, or “facets”

• The general form for putting a restriction on a text value is: - <xs:element name=" name "> (or xs:attribute ) <xs:restriction base=" type "> ... the restrictions ... </xs:restriction> </xs:element>
• For example:
  • <xs:element name="age"> <xs:restriction base="xs:integer"> <xs:minInclusive value="0"> <xs:maxInclusive value="140"> </xs:restriction> </xs:element> Docsity.com

Restrictions on strings

• length -- the string must contain exactly value characters
• minLength -- the string must contain at least value characters
• maxLength -- the string must contain no more than value characters
• pattern -- the value is a regular expression that the string must match
• whiteSpace -- not really a “restriction”--tells what to do with whitespace - value="preserve" Keep all whitespace - value="replace" Change all whitespace characters to spaces - value="collapse" Remove leading and trailing whitespace, and Docsity.com

Enumeration

• An enumeration restricts the value to be one of a fixed set of values
• Example:
  • <xs:element name="season"> <xs:simpleType> <xs:restriction base="xs:string"> <xs:enumeration value="Spring"/> <xs:enumeration value="Summer"/> <xs:enumeration value="Autumn"/> <xs:enumeration value="Fall"/>Docsity.com

Global and local definitions

• Elements declared at the “top level” of a are available for use throughout the schema
• Elements declared within a xs:complexType are local to that type
• Thus, in <xs:element name="person"> <xs:complexType> <xs:sequence> <xs:element name="firstName" type="xs:string" /> <xs:element name="lastName" type="xs:string" /> </xs:sequence> </xs:complexType> </xs:element> the elements firstName and lastName are only locally declared
• The order of declarations at the “top level” of a do not specify the order in the XML data document Docsity.com

Declaration and use

• So far we’ve been talking about how to declare types, not how to use them
• To use a type we have declared, use it as the value of type="..." - Examples: - <xs:element name="student" type="person"/> - <xs:element name="professor" type="person"/> - Scope is important: you cannot use a type if is local to some other type

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xs:all

• xs:all allows elements to appear in any order
• <xs:element name="person"> <xs:complexType> <xs:all> <xs:element name="firstName" type="xs:string" /> <xs:element name="lastName" type="xs:string" /> </xs:all> </xs:complexType> </xs:element>
• Despite the name, the members of an xs:all group can occur once or not at all
• You can use minOccurs="0" to specify that an element is optional (default value is 1 ) - In this context, maxOccurs is always (^1) Docsity.com

Referencing

• Once you have defined an element or attribute (with name="..."), you can refer to it with ref="..."
• Example:
  • <xs:element name="person"> <xs:complexType> <xs:all> <xs:element name="firstName" type="xs:string" /> <xs:element name="lastName" type="xs:string" /> </xs:all> </xs:complexType> </xs:element>
  • <xs:element name="student" ref="person">
  • Or just: <xs:element ref="person"> Docsity.com