XML Schema Definition Language XSD Jan 14 2019 XML Schemas Schemas is a general term DTDs are a form of XML schemas When we say XML Schemas we usually mean the W3C XML Schema Language According to the dictionary a schema is a structured framework or plan 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 2 Why XML Schemas DTDs provide a very weak specification language DTDs are written in a strange non XML format You can t put any restrictions on text content You have very little control over mixed content text plus elements You have little control over ordering of elements You need separate parsers for DTDs and XML The XML Schema Definition language solves these problems XSD gives you much more control over structure and content XSD is written in XML 3 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 nonintuitive and confusing Nevertheless XSD is not likely to go away quickly 4 Referring to a schema To refer to a DTD in an XML document the reference goes before the root element xml version 1 0 DOCTYPE rootElement SYSTEM url rootElement rootElement To refer to an XML Schema in an XML document the reference goes in the root element xml version 1 0 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 rootElement 5 The XSD document Since the XSD is written in XML it can get confusing which we are talking about Except for the additions to the root element of our XML data document the rest of this lecture is about the XSD schema document The file extension is xsd The root element is schema The XSD starts like this xml version 1 0 xs schema xmlns xs http www w3 rg 2001 XMLSchema 6 schema The schema element may have attributes xmlns xs http www w3 org 2001 XMLSchema 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 7 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 contain text other elements or both text and other elements 8 Defining a simple element A simple element is defined as xs element name name type type where name is the name of the element the most common values for type are xs boolean xs integer xs date xs string xs decimal xs time Other attributes a simple element may have default default value fixed value if no other value is specified no other value may be specified 9 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 fixed value no other value may be specified use optional the attribute is not required default use required the attribute must be present 10 Restrictions or facets The general form for putting a restriction on a text value is xs element name name xs restriction base type the restrictions xs restriction xs element or xs attribute For example xs element name age xs restriction base xs integer xs minInclusive value 0 xs maxInclusive value 140 xs restriction xs element 11 Restrictions on numbers minInclusive number must be the given value minExclusive number must be the given value maxInclusive number must be the given value maxExclusive number must be the given value totalDigits number must have exactly value digits fractionDigits number must have no more than value digits after the decimal point 12 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 replace all sequences of whitespace with a single space 13 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 xs enumeration value Winter xs restriction xs simpleType xs element 14 Complex elements A complex element is defined as xs element name name xs complexType information about the complex type xs complexType xs element Example 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 xs sequence says that elements must occur in this order Remember that attributes are always simple types 15 Global and local definitions Elements declared at the top level of a schema 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 schema do not specify the order in the XML data document 16 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 17 xs sequence We ve already
View Full Document
Unlocking...