Towards a minimal set of operations for nested relations
Marc H. Scholl · KOPS (University of Konstanz) · 1987
Since the first publications on non-first-normal-form relations in the late 70's and early 80's, a variety of formalizations of the data structure of and operations for nested relations have been devised. The data structure itself is defined almost identical in the several approaches, only some subtle difi'erences concerning special cases can be observed. For instance, both the VER50 relations [AB84J and the PNF relations of [RKS85J do not allow relations without an "atomic key", i.e. the set of attributes forming a key must not contain relation-valued attributes, which, however, is allowed in [5584/86J. However, concerning operations for nested relations, a much broader scale of languages was proposed. Formal operations were defined in algebra, calculus and SQL style. In this position paper we want to compare several proposals of algebras for nested relations. We distinguish between "minimal " extensions of the fiat algebra (as for example the ones given by [FT83] or [RKS85J)-i.e. those that try to get along with the "Nest" and "Unnest " operations-, and the "maximal " extensions that supply nested operations (e.g. [AB84, 5S84/86]). Others have investigated equivalences between algebras and calculi for nested relations, e.g. [RK585J, van Gucht, Abiteboul and Beeri (the latter ones see this workshop). This issue is