On the decomposition of join dependencies
Marc Gyssens, Jan Paredaens · 1984
In [9] we proposed a method for decomposing join dependencies (jd's) in a relational database. Decomposing a jd can be useful for separating cyclic and acyclic parts of jd's, obtaining more insight in the structure of a jd or making integrity-checking more efficient. The decomposition methodology of [9] has many desirable properties. However, in general it cannot generate all the decompositions of a given jd. In this paper, we first recall this decomposition methodology and its most important properties. We then introduce a subclass of jd's, the unambiguous jd's. We show that this class represents exactly those jd's that have a unique decomposition (which can be obtained by our method). We also give a characterization of this decomposition in terms of the structure of the original jd. To prove our results, we make extensive use of hypergraph theory.