Affiliations: [a] Department of Computer Science, University College, University of London, Cower Street, London, WC1E 6BT, U.K. | [b] Department of Computer Science, Birkbeck College, University of London, Malet Street, London, WC1E 7HX, U.K.
Abstract: The nested relational model extends the fiat relational model by relaxing the first normal form assumption in order to allow the modelling of complex objects. Recently many extended algebras have been suggested for the nested relational model, but only few have incorporated null values into the attribute domains. Furthermore, some of the previously defined extended algebras are defined only over a subclass of nested relations, and all of them are difficult to use, since the user must know the detailed structure of the nested relations being queried. Herein, we define an extended algebra for nested relations, which may contain null values, called the null extended algebra. The null extended algebra is defined over the general class of nested relations with null values and, in addition, allows queries to be formulated without the user having to know the detailed structure of the nested relations being queried. In this sense, our null extended join operator of the null extended algebra is unique in the literature, since it joins two nested relations by taking into account all their common attributes at all levels of their structure, whilst operating directly on the two nested relations. All the operators of the null extended algebra are proved to be faithful and precise. The null extended algebra is a complete extended algebra in the context of nested relations, and, in addition, it includes the null extended powerset operator, which provides recursion and iteration facilities.
