Joining nested relations and subrelations

Abstract This paper proposes a generalisation of the “natural” join operation which is shown to be applicable to all joinable nested relations. The “natural” join operation can be applied to any pair of relations which have one or more attributes in common even when they are in different subrelations and at different nesting levels in each relation; it is not just executing a join between specific user-defined attributes. This paper systematically reviews the varied forms of joins between nested relations and subrelations. The generalisation of join uses one or more of the six distinct cases of the join operation applied to nested relations which are identified later in the paper, distinguished by certain properties of the attributes in the join paths between the relations that participate in the join operation. These properties depend on whether an attribute is either atomic or relation-valued and on whether it is at either the top level or lower levels (same or different) of the two relations.