Computable queries for relational data bases

The concept of a "reasonable" query in a relational data base is investigated. We provide an abstract characterization of the class of queries which are computable, and define the completeness of a query language as the property of being precisely powerful enough to express the queries in this class. Our main result is the completeness of a simple programming language which can be thought of as consisting of the relational algebra augmented with the power of iteration.