An XML Algebra for XQuery

An XML algebra supporting the XQuery query language is presented. The usage of expression constructing operators instead of high-order operations using functions as parameters has permitted us to remain in the limits of first-order structures whose instance is a many-sorted algebra. The set of operators of the presented algebra substantially differs from the set of operators of relation algebra. It is caused by the complex nature of the XML data model comparing with relational one. Actually, only predicative selection is more or less same in both algebra. Yet, the XML algebra in addittion permits selection by node test. The relational projection operator is replaced by the path expression and navigating functions; the join operator is replaced by unnesting join expressions. In addition, a number of node constructing expressions permitting update of the algebra state are defined.