XML Data Management: Native XML and XML-Enabled Database Systems

2.3. XML Storage

XML documents can have some schematic information (e.g., in the form of a DTD or W3C XML Schema), but they are not required to. Even if a schema exists, comments and processing instructions may occur at any place without previous declaration in the schema. Thus, the classical database approach of handling objects of a predefined type cannot be applied to the storage of XML. It is mandatory that schemas remain optional. Schemas may also be partial (i.e., describe only parts of the data, as discussed later in this chapter), and they can be easily modified even for existing data.

The descriptive power of DTDs is not sufficient for database purposes. For example, DTDs lack information such as data type, which is needed for the proper ...