Standardizing the Querying Process with SGML
				The SQL DTD

				Arijit Sengupta

Abstract

One of the most exciting applications of SGML which has emerged in the
recent years is its use in document databases. The structural
information embedded in SGML documents makes it possible to query SGML
documents and extract information in an automatic manner; however,
this querying process has not been standardized. As a result,
different SGML database implementations use their own query language
syntax, thus making the migration from one system to another a
difficult process. In the relational database domains, however, the
query language SQL (Structured Query Language) has been a standard for
over ten years and is universally used in most relational database
systems. Although originally designed for relational databases, SQL is
quite powerful for specifying complex queries in a relatively
easy-to-understand syntax. With a small set of extensions to take
advantage of the hierarchical structure of SGML, SQL can be easily
adapted for use with SGML document databases.

The powerful "generalized" nature of SGML makes it easy to implement
SQL as an SGML DTD (Document Type Definition) , so that queries can be
expressed as document instances of the SQL DTD. Current SGML authors
and users can write queries expressed in this DTD without learning a
different language or using a separate editor. Moreover, because of
the portable nature of SGML, these queries can be used in any SGML
database system and can be converted to regular SQL for use in a
relational or Object-Relational/Object-Oriented database system, if
necessary. Databases that support the SQL DTD can also store the
queries without any extra effort, and subsequently query them for
inferring optimization parameters.

 This paper presents a representative DTD for the SQL query language,
with extensions for use with hierarchically structured documents. It
also compares this language with languages proposed and implemented,
including SDQL (Standard Document Query Language) - the query
language in the DSSSL standard. This paper explains the
advantages of using this language as a query language in document
database systems and the necessity for standardizing the querying
process in document databases. Finally, it discusses some
implementation issues and complexity measures.