On query-answering in relational data bases

In recent years the relational model has been widely adopted for data base description. According to this model a data base, DB, describes certain objects of the world having certain attributes, and the relationships among them. Thus, DB is characterized by a set of attributes, D , a set of domains associated with the attributes, and a set of dependencies, F , corresponding to the relationships among the attributes (all the terms and concepts not defined here are those of References 1 – 3 ). A data base is a collection of relations, R ={ R i }. Each relation R i is characterized by a set of attributes S i ={ D j | D j ∈ D } called its scheme , and consists of a set of tuples. Each tuple is a map from the attributes of the relation scheme to their domains that satisfies all the dependencies of F (we shall consider functional dependencies).