{"id":71,"date":"2018-07-26T06:28:15","date_gmt":"2018-07-26T06:28:15","guid":{"rendered":"http:\/\/csp12.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=71"},"modified":"2018-08-02T05:51:38","modified_gmt":"2018-08-02T05:51:38","slug":"xmlschemapart-i","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/csp12\/chapter\/xmlschemapart-i\/","title":{"rendered":"XMLSCHEMA(PART I)"},"content":{"raw":"
\r\n\r\n\u00a0 \u00a0 \u00a0XML Schemas<\/strong>\r\n\r\n \r\n

\u201cSchemas\u201d is a general term used in many context and DTD's are a form of XML schemas. According to the dictionary, a schema is \u201ca structured framework or plan\u201d. When we say \u201cXML Schemas,\u201d we usually mean the W3C XML Schema Language. This is also known as \u201cXML Schema Definition\u201d language, or XSD.An XML Schema consists of components such as type definitions and element declarations that can be used to assess the validity of well-formed element and attribute information items in the file.<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n\u00a0 <\/strong>\r\n\r\nWhy we need XML Schemas?<\/strong>\r\n\r\n \r\n

Some of the reasons why we need XML schemas are listed below,DTDs provide a very weak specification language. We can\u2019t put any restrictions on text content and we have very little control over mixed content (text plus elements). Moreover,we have little control over ordering of elements.<\/p>\r\n \r\n

DTDs are written in a strange (non-XML) format and so we need separate parsers for DTDs and XML . The XML Schema Definition (XSD) language solves these problems by giving us much more control over structure and content. XSD is written in XML.<\/p>\r\n \r\n\r\nWhy not XML schemas?<\/strong>\r\n\r\n \r\n

Even though XML schemas supports many features that are not in DTD, DTD's are widely used than Schemas. DTD's have been around longer than XSD and 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 for the reasons what has been explained earlier.<\/p>\r\n \r\n\r\nReferring to a schema<\/strong>\r\n\r\n \r\n\r\nTo refer to a DTD in an XML document, the reference goes before the root element as given below,\r\n\r\n<?xml version=\"1.0\"?>\r\n\r\n<!DOCTYPE rootElement SYSTEM \"url\">\r\n\r\n<rootElement> ... <\/rootElement>\r\n\r\n \r\n

To refer to an XML Schema in an XML document, the reference goes in the root element as shown below,<\/p>\r\n

<?xml version=\"1.0\"?><\/p>\r\n

<rootElement xmlns:xsi=\"http:\/\/www.w3.org\/2001\/XMLSchema-instance (The XML Schema Instance reference is required) xsi:noNamespaceSchemaLocation=\"url.xsd\"><\/p>\r\n

(This is where the XML Schema definition can be found) ...<\/p>\r\n<\/rootElement>\r\n\r\n<\/div>\r\n\u00a0 \u00a0 \u00a0The XSD document<\/strong>\r\n

\r\n\r\n \r\n\r\nThe XSD is written in XML and the file extension is .xsd. The root element is <schema>.\r\n\r\n \r\n\r\nThe XSD starts like this:\r\n\r\n<?xml version=\"1.0\"?>\r\n\r\n<xs:schema xmlns:xs=\"http:\/\/www.w3.rg\/2001\/XMLSchema\">\r\n\r\n \r\n\r\n<schema><\/strong>\r\n\r\n \r\n

The root element <schema><\/strong> may have attributes, one of which is including the namespace which is necessary to use the predefined schema elements defined in the namespace.<\/p>\r\n \r\n

xmlns:xs= \u201chttp:\/\/www.w3.org\/2001\/XMLSchema<\/a>\"<\/p>\r\n \r\n

This statement is necessary to specify where all our XSD tags are defined.<\/p>\r\n

elementFormDefault=\"qualified\"<\/p>\r\n \r\n

This means that all XML elements must be qualified which means it uses a namespace.It is highly desirable to qualify all elements, or problems will arise when another schema is added.<\/p>\r\n \r\n\r\nSample Schema header<\/strong>\r\n\r\n \r\n\r\nThe <schema> element may contain some more attributes. A schema declaration often looks something like this:\r\n\r\n \r\n\r\n<?xml version=\"1.0\"?>\r\n\r\n<xs:schema xmlns:xs=\"http:\/\/www.w3.org\/2001\/XMLSchema\"\r\n\r\ntargetNamespace=\"http:\/\/www.w3schools.com\" xmlns=\"http:\/\/www.w3schools.com\"\r\n\r\nelementFormDefault=\"qualified\">\r\n\r\n... ...\r\n\r\n<\/xs:schema>\r\n\r\n \r\n\r\nThe following fragment,\r\n\r\n\u2013\u00a0\u00a0 xmlns:xs=http:\/\/www.w3.org\/2001\/XMLSchema<\/a>\r\n\r\n \r\n

indicates that the elements and data types used in the schema (i.e., schema, element, complexType, sequence, string, boolean, etc.) come from the \"http:\/\/www.w3.org\/2001\/XMLSchema\" namespace. It also specifies that the elements and data types that come from the \"http:\/\/www.w3.org\/2001\/XMLSchema\" namespace should be prefixed with 'xs:'.<\/p>\r\n \r\n

The following fragment,\u00a0\u00a0targetNamespace=http:\/\/www.w3schools.com<\/a><\/p>\r\n

indicates that the elements defined by this schema (note, to, from, heading, body.) come from the \"http:\/\/www.w3schools.com\" namespace.<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n\u00a0 \u00a0The following fragment,\r\n\r\n-\u00a0 xmlns=http:\/\/ <\/span>www.w3sch ools.c om<\/a>\r\n\r\nindicates that the default namespace is \"http:\/\/www.w3schools.com\".\r\n\r\n \r\n\r\n \r\n\r\nThe following fragment,\r\n\r\n\u2013\u00a0\u00a0\u00a0 elementFormDefault=\"qualified\u201c\r\n\r\n \r\n\r\nindicates that any elements used by the XML instance document which were declared in this schema must be namespace qualified.\r\n\r\n \r\n\r\nAn XML Instance Document Example is given below,\r\n\r\n<book isbn=\u201c012345\">\r\n\r\n<title> XML<\/title>\r\n\r\n<author>CSXX<\/author>\r\n\r\n<qualification> XXXXXX <\/qualification>\r\n\r\n<\/book>\r\n\r\n \r\n\r\nThe Schema for the above XML file could be written as follows,\r\n\r\n<?xml version=\"1.0\" encoding=\"utf-8\"?>\r\n\r\n<xs:schema xmlns:xs=\"http:\/\/www.w3.org\/2001\/XMLSchema\"> <xs:element name=\"book\">\r\n\r\n<xs:com plexType>\r\n\r\n<xs:sequence>\r\n\r\n<xs:element name=\"title\" type=\"xs:string\"\/> <xs:element name=\"author\" type=\"xs:string\"\/> <xs:element name=\u201cqualification\u201c type=\u201cxs:string\u201d\/>\r\n\r\n<\/xs:sequence>\r\n\r\n<\/xs:com plexType>\r\n\r\n<\/xs:element>\r\n\r\n<\/xs:schema>\r\n\r\nLet us learn about how to write XML schemas in the forth coming sections.\r\n\r\n<\/div>\r\n\u00a0 \u00a0 \u00a0What\u2019s in a Schema?<\/strong>\r\n
\r\n\r\n \r\n

A Schema is an XML document but a DTD is not an XML document. Because a schema is an XML document, it must have a root element and the root element is <schema> for any XSD file. Within the root element, there can be any number and combination of Inclusions, Imports, Re-definitions and Annotations. This is followed by any number and combinations of Simple and complex data type definitions, Element and attribute definitions, Model group definitions and Annotations.<\/p>\r\n \r\n\r\nStructure of a Schema<\/strong>\r\n\r\n \r\n\r\n<schema><\/strong>\r\n\r\n<!\u2013 any number of the following --><\/strong>\r\n\r\n<include ...\/>\r\n\r\n<import> ... <\/import>\r\n\r\n<redefine> ... <\/redefine>\r\n\r\n<annotation> ... <\/annotation>\r\n\r\n<!\u2013 any number of following definitions --><\/strong>\r\n\r\n<simpleType> ... <\/simpleType>\r\n\r\n<com plexType> ... <\/com plexType>\r\n\r\n<element> ... <\/element>\r\n\r\n<attribute\/>\r\n\r\n<attributeGroup> ... <\/attributeGroup>\r\n\r\n<group> ... <\/group>\r\n\r\n<annotation> ... <\/annotation>\r\n\r\n<\/schema><\/strong>\r\n\r\n \r\n\r\nXML Schema Components<\/strong>\r\n