This paper talks about 3 main points. First, the features required in an XML query engine. Second, how XML-QL, an XML query language, satisfies some of these features. Finally, the paper mentions briefly some other languages with summaries describing which features they satisfy and which ones they don't.
In describing the features needed in an XML query engine, the authors gave a very comprehensive overview. They stated 14 features; all of them were well justified. But what I really found interesting were features 9 and 14. Feature 9 talks about embedding XML data in a query and embedding a query in XML data. The latter part of the feature is what I think is really interesting and very useful, because it may reduce data redundancy. Feature 14 requires that an XML query language to have the capability of being presented as XML data, which, in my opinion, is an important feature. Although it may seem to be trivial and worth noting and including in an XML query language, it seems from table 1 in the paper that it has been overlooked by most of XML query language designers, except for XSL query language.
A feature that the authors overlooked is the comparison complexity. An XML query language should be able to distinguish between comparing letters and comparing numerals in predicates of queries.
They also overlooked the ability to aggregate XML documents or elements of XML documents and querying them in one step. The authors did not state this feature. However, in feature 1, the authors mentioned combination, which is different from aggregation, in a sense that combination is more like a join from an SQL point of view.
The example the authors chose to demonstrate different features needed in an XML query language was XML-QL. This is not a surprise since the authors of this paper wrote also XML-QL. But yet, the choice of such an example turned out to be an excellent one, since XML-QL satisfies most of their stated features more than any other query language. An easier to understand query language would have been a better choice. The way they demonstrated different features was clear and concise. They gave very good examples on each and every feature met by XML-QL.
One of the best things in this paper, at least in my opinion, is table 1. This table shows different query languages and whether or not they satisfy each of the 14 features. Composing such a table requires lots of work and knowledge. This table is a good reference for choosing an XML query language for specific applications that require certain capabilities in a language.
Overall, the paper is very well written and explained. It explores almost every aspect of querying XML. I am in no position to judge or criticize the knowledge of the authors. What I stated above were mere thoughts and questions that popped into my head when I read their fine paper.
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