Summary of XML

Summary

• Like the Hyper-Text Markup Language, HTML, on which the Web is based, the Extensible Markup Language, XML, is a descendant of the Standard Generalized Markup Language (SGML). XML was originally intended for providing functional markup for Web documents, but has now become the defacto standard data format for data exchange between applications.

• XML documents contain elements, with matching starting and ending tags indicating the beginning and end of an element. Elements may have subelements nested within them, to any level of nesting. Elements may also have attributes. The choice between representing information as attributes and sub- elements is often arbitrary in the context of data representation.

• Elements may have an attribute of type ID that stores a unique identifier for the element. Elements may also store references to other elements using attributes of type IDREF. Attributes of type IDREFS can store a list of references.

• Documents may optionally have their schema specified by a Document Type Declaration, DTD. The DTD of a document specifies what elements may occur, how they may be nested, and what attributes each element may have.

• Although DTDs are widely used, they have several limitations. For instance, they do not provide a type system. XMLSchema is a new standard for specifying the schema of a document. While it provides more expressive power, including a powerful type system, it is also more complicated.

• XML data can be represented as tree structures, with nodes corresponding to elements and attributes. Nesting of elements is reflected by the parent-child structure of the tree representation.

• Path expressions can be used to traverse the XML tree structure, to locate required data. XPath is a standard language for path expressions, and allows required elements to be specified by a file-system-like path, and additionally allows selections and other features. XPath also forms part of other XML query languages.

• The XSLT language was originally designed as the transformation language for a style sheet facility, in other words, to apply formatting information to XML documents. However, XSLT offers quite powerful querying and transformation features and is widely available, so it is used for quering XML data.

• XSLT programs contain a series of templates, each with a match part and a select part. Each element in the input XML data is matched against available templates, and the select part of the first matching template is applied to the element.

Templates can be applied recursively, from within the body of another template, a procedure known as structural recursion. XSLT supports keys, which can be used to implement some types of joins. It also supports sorting and other querying facilities.

• The XQuery language, which is currently being standardized, is based on the Quilt query language. The XQuery language is similar to SQL, with for, let, where, and return clauses.

However, it supports many extensions to deal with the tree nature of XML and to allow for the transformation of XML documents into other documents with a significantly different structure.

• XML data can be stored in any of several different ways. For example, XML data can be stored as strings in a relational database. Alternatively, relations can represent XML data as trees. As another alternative, XML data can be mapped to relations in the same way that E-R schemas are mapped to relational schemas.

XML data may also be stored in file systems, or in XML-databases, which use XML as their internal representation.

• The ability to transform documents in languages such as XSLT and XQuery is a key to the use of XML in mediation applications, such as electronic business exchanges and the extraction and combination of Web data for use by a personal finance manager or comparison shopper.