Derivation of Constraints and Database Relations

In this paper we investigate which constraints may be derived from a given set of constraints. We show that, given a set of relations R, all of which are invariant under some set of permutations P, it is possible to derive any other relation which is invariant under P, using only the projection, Cartesian product, and selection operators, together with the eeective domain of R, provided that the eeective domain contains at least three elements. Furthermore, we show that the condition imposed on the eeective domain cannot be removed. This result sharpens an earlier result of Paredaens 13], in that the union operator turns out to be superruous. In the context of constraint satisfaction problems, this result shows that a constraint may be derived from a given set of constraints containing the binary disequality constraint if and only if it is closed under the same permutations as the given set of constraints.