Introduction

Beyond the basic concepts that replicate the well-known modelling features inherited from XML DTDs, the XML Schema standard includes several entirely new features, and resurrects some of the advanced features found in SGML DTDs. These features give the schema author the ability to:

• define default and fixed element content values

• define a content model that requires all of the referenced elements to be present, but does not dictate the order in which they can occur

• add attributes to simple element types

• explicitly identify element instances that have no value

• define context-specific elements within other elements, possibly with non-unique names, but with different content models and attributes

• specify that elements must have unique identifier values, possibly involving combinations of element content and attribute values

• define unique keys from attribute values, element content or combinations of attributes and elements, and validate that specific instances exist from references elsewhere in the same document

• choose a data type for an attribute value or for element content, from a large range of options

• reference data type libraries

• create new simple data types from existing types

• use complex 'patterns' to precisely control attribute and element values

• create new complex data types from existing complex data types

• restrict derivations of data types and usage of base or derived types

• re-define definitions in external schema documents, and use definitions created for other target namespaces.