On the expressive power of query languages

Abstract

Two main topics are addressed. First, an algebraic approach is presented to define a general notion of expressive power. Heterogeneous algebras represent information systems and morphisms represent the correspondences between the instances of databases, the correspondences between answers, and the correspondences between queries. An important feature of this new notion of expressive power is that query languages of different types can be compared with respect to their expressive power. In the case of relational query languages, the new notion of expressive power is shown to be equivalent to the notion used by Chandra and Harel. In the case of nonrelational query languages, the versatility of the new notion of expressive power is demonstrated by comparing the fixpoint query languages with an object-oriented query language called FQL. The expressive power of the Functional Query Language FQL is the second main topic of this paper. The specifications of FQL functions can be recursive or even mutually recursive, FQL has a fixpoint semantics based on a complete lattice consisting of bag functions. The query language FQL is shown to be more expressive than the fixpoint query languages. This result implies that FQL is also more expressive than Datalog with stratified negation. Examples of recursive FQL functions are given that determine the ancestors of persons and the bill of materials.

Semantics and Expressive Power of Subqueries and Aggregates in SPARQL 1.1

“…Assume a very restricted use of Extend where only a Boolean expression that always evaluates to false is allowed; although a query could still introduce a fresh value not occurring in the graph, this is done in a trivial way and hence one could argue that there is no actual gain in expressive power in this case. We next introduce a more liberal notion of expressive power derived from [3,37]. This notion is based on a generalisation of query equivalence which allows for changes in the input graph; these changes are, however, far from arbitrary and need to be uniform across all queries.…”

Relationship-based access control policies and their policy languages

“…As demonstrated in Section 4.3, the inclusion of FR in OC is proper. It is desirable if one can close the gap, and provide a precise identification of the family of policies definable in E. We anticipate that such an effort can benefit from the application of techniques from Finite Model Theory [28], which has long been applied to understand the expressiveness of database query languages [11,10,34].…”

XQuery layers

“…the one used in [11]. [13] <computation> ::= an arbitrary arithmetic expression of <variable>s and <const>s [14] <aggregate> ::= the invocation of an aggregate function The extension from CoreXQBE to FullXQBE requires extending four productions and adding four more productions, as described in Figure 4.…”

“…Our approach to the stratification of XQuery is inspired to previous similar works dedicated to SQL, see [13,14].…”