Review - Xml-ql: A Query Language for XML.

Review

This paper describes XML-QL, a query language for XML. It also describes a data model for XML. XML-QL proposes to overcome the fundamental database problems of data extraction, integration, transportation and transformation. The presentation is clear and precise. It gives a good overview of the capabilities of XML-QL. The examples used helped clarify the points made, but in certain cases were slightly unclear (some of the variable bindings were not easily comprehensible). Following are a list of technical strengths and weaknesses of XML-QL as I see it.

Strengths:

• The use of the familiar SELECT-WHERE type of construct makes it easy to understand. All the basic query operations, like selection, extraction etc. can be rolled into a single query command. Further, since the output of any query can be constructed as new XML data, an application need not worry whether it is looking at a data source or query result. It can also query multiple documents simultaneously and produce integrated views of their data. These are features that are important for a good query language.
• XML-QL supports two distinct data models: an unordered one and an ordered one. This offers a degree of flexibility.
• The presence of DTD is optional. This feature is important because not all XML documents need have a DTD and the query language should be able to effectively extract data from such documents too. It supports querying of element tags using tag variables. It also allows specification of regular path expressions for recursive queries. These features enable the use of a single query on different documents that have similar data but different DTDs. This aspect is key for EDI (Electronic Data Interchange), which is one of the most important applications of XML.

• The abbreviation of the end tags from </element> to </>, while making the queries brief and enables the use of tag variables, removes a small percent of readability.

Copyright © 2000 by the author(s). Review published with permission.

References

[1]

Alin Deutsch, Mary F. Fernandez, Daniela Florescu, Alon Y. Levy, Dan Suciu: Xml-ql: A Query Language for XML. http://www.w3.org/TR/NOTE-xml-ql/ BibTeX