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structures as granules for the use of functionalities of DBMSs is an important
research issue. If XML documents are stored in relational or object-oriented
databases, the development of algorithms for translating queries in XML
query languages into SQL or OQL is also a mandatory technical issue.
2.
XML as a common data model on the Web:
A huge amount of information in various format is available on the Web.
While multimedia data is a powerful means to convey information intuitively,
a significant part of intellectual information on the Web is presented in forms
of text. Some of those data are stored in relational or object-oriented databases,
while some others are stored in HTML or plain text files.
If there is a common data model which is capable of representing such
wide variety of text based data, we could transform existing data on the
Web into the common data model. The transformed data does not need to
be materialized, but could be a view of existing data. With the existence of
a common data model, data on the Web becomes uniformly accessible, and
thus interchange and distribution of information is greatly facilitated. XML
is expected to become such a common data model on the Web. Here, a
natural and important question is: “Does XML have enough power to
express wide variety of existing information resources on the Web?” The
XML1.0 data structure has enough expressive power in that it can simulate
relational data model and a significant part of object-oriented data model.
Also, HTML is a language whose data structure could be defined by XML
DTD. However, the current XML1.0 lacks of data types such as integer,
real and date. A set of extensive data types will be supported in schema
languages under development such as XML Schema
20,
21)
and RELAX
13)
.
By transforming existing data into XML, the Web can be regarded as a
huge, highly-distributed XML database. The notion of “database” here is
not same as those of traditional DBMSs. Databases in this broader sense
means a collection of physical or virtual XML documents.
2
XML1.0
XML1.0
23)
is a core standard in the family of XML related standards. To put it
simply, a DTD of XML1.0 is a context-free grammar, and an XML document
conforming to the DTD is a parse tree. XML Information Set
27)
describes an abstract
information set available in an XML document. Also, DOM (Document Object
Model)
22)
, an API (Application Programming Interface) for XML documents, and
XPath (XML Path Language)
26)
respectively defines their own data models. In this
paper, we will follow the XPath data model. Wadler gave a formal description of
the XPath data model
17)
.
For example, the XML document in Figure 1 can be represented as a tree in
