The theory of joins in relational databases

Answering queries in a relational database often requires that the natural join of two or more relations be computed. However, the result of a join may not be what one expects. In this paper we give efficient algorithms to determine whether the join of several relations has the intuitively expected value (is lossless ) and to determine whether a set of relations has a subset with a lossy join. These algorithms assume that all data dependencies are functional. We then discuss the extension of our techniques to the case where data dependencies are multivalued.