Null values in nested relational databases

The desire to extend the applicability of the relational model beyond traditional data-processing applications has stimulated interest in nested or non-first normal form relations in which the attributes of a relation can take on values which are sets or even relations themselves. In this paper, we study the role of null values in the nested relational model using an open world assumption. We extend the traditional theory and study the properties of extended operators for nested relations containing nulls. The no-information, unknown, and non-existent interpretation of nulls are discussed and the meaning of “empty set” is clarified. Finally, contrary to several previous results, we determine that the traditional axiomatization of functional and multivalued dependencies is valid in the presence of nulls.