A note on lossless database decompositions

It is known that under a wide variety of assumptions a database decomposition is lossless if and only if the database scheme has a lossless join. Biskup, Dayal and Bernstein (1979) have shown that when the given dependencies are functional, the database scheme has a lossless join if and only if one of the relation schemes is a key for the universal scheme. In this note we supply an alternative proof of that characterization. The proof uses tools from the theory of embedded join dependencies and the theory of tuple and equality generating dependencies, but is, nevertheless, much simpler than the previously published proof. © 1984.