What are XML Schemas?

• Like DTDs, schemas are another way to define the elements of an XML document.

• The following example shows a DTD and an equivalent Schema:

  DTD:

  <!ELEMENT book (title, author, publisher?)> <!ELEMENT title (#PCDATA)> <!ELEMENT author (#PCDATA)> <!ELEMENT publisher (#PCDATA)>

  Schema:

  <?xml version = "1.0"?> <Schema xmlns="urn:schemas-microsoft-com:xml-data"> <ElementType name="book" content="eltOnly" order="seq"> <element type="title" /> <element type="author" /> <element type="publisher" minOccurs="0" /> </ElementType> <ElementType name="title" content="textOnly" /> <ElementType name="author" content="textOnly" /> <ElementType name="publisher" content="textOnly" /> </Schema>

• How do schemas differ from DTDs?
  • They use XML syntax.
  • You can define element and attribute data types.
  • You can define open content models.
  • They allow a slightly greater flexibility for defining content models.

• Schemas are likely to replace DTDs in the future.

• To associate a schema to an XML document, use the xmlns (XML namespace) attribute on the document's root element:

  <?xml version = "1.0"?> <catalog xmlns="x-schema:schema3.xml">

  Note: although namespaces are in fact only unique identifiers and do not formally imply conformance to any schema or DTD, IE5 uses this method to associate schemas to documents.

• Schema files use XML syntax. They must begin with the Schema tag:

  <?xml version = "1.0"?> <Schema xmlns="urn:schemas-microsoft-com:xml-data"> . . . schema definitions go here . . . </Schema>

• The xmlns attribute defines the namespace for schemas. This simply means that the language for defining schemas is made up of predefined tags, which belong to that namespace.