Joining nested relations and subrelations

This paper proposes a generalisation of the "natural Join operation which i s shown to be applicable to all joinable nested relations. The "natural" jo in 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 d ifferent nesting levels in each relation; it is not just executing a join b etween 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 wh ich 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 e ither atomic or relation-valued and on whether it is at either the top leve l or lower levels (same or different) of the two relations. (C) 2000 Elsevi er Science Ltd. All rights reserved.