Since the World Wide Web Consortium (W3C) has recommended XQuery as the standard XML query language, the interest in using existing relational technology to query the XML data has dramatically increased. The most significant challenge of the relational approach is how to fully support XQuery semantics in XQuery-to-SQL translation. To eliminate the implicit semantics of XQuery, an XQuery fragment must be defined with simple syntax and explicit semantics. XQ is proposed as an XQuery fragment to express XML queries.
In this thesis, XQ is intensively investigated. It is encoded by System F, a second-order lambda calculus with a considerable expressive power and a strong normalization property. Since XML data is defined as inductive data types, XML tree and XML forest, in System F, all basic XML operators in XQ have been successfully encoded. Also, the semantics of XQ are represented in System F where XQ;;s semantics environment is encoded by an Environment data type with the corresponding operators. The successful encoding of XQ by System F ensures the termination of XQ query evaluation.
Moreover, an extension of XQ by a new tree operator Xtree and a vertical Vfor clause is proposed in this thesis to express some undefinable XQ queries. It is demonstrated that this extension still allows XQ to retain its XQ-to-SQL translation property that ensures the polynomial evaluation time complexity, and its System F encodable property that ensures the termination of query evaluation.
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