A view of directions in relational database theory [chapter]

Jeffrey D. Ullman
1981 Lecture Notes in Computer Science  
We shall briefly survey what the author believes are some of the most fruitful directions in relational database theory. These directions include dependency inferences, support for the universal relation concept, null value semantics, and an exploration of the properties of acyclic database schemes. I. Introduction We shall assume the reader is familiar with the basic concepts of relational database theory, at least at the level covered by [U] or [BBG]. These concepts include 3. Functional
more » ... dency (FD). A functional dependency X --~ Y is an assertion about a relation scheme. It asserts of any "legal" relation that two of its tuples t and s that agree on set of attributes X (written t[X] -~ sIX]) also agree on Y. A relation with that property is said to satisfy the dependency. (MVD). A multivalued dependency X -~* Y is, very informaUy, a statement that there is a set of Y-values associated with each X-value. MuItivatued Dependency More formally, if t and s are two tuples of a relation that satisfies X -+~ Y, and t[X] ~-siX], then the relation must have a third tuple u that takes the Y-value from one of the tuples, say t, and its value everywhere else from the other. That is, if Z is all attributes not in X Y , then u[Y] ~-t[Y] and u[XZ] -~ s[XZ] . 5. Normalization. Certain relation schemes have redundancy, in the sense that predictable values appear in certain tuples of the relations over that scheme. It is the dependencies, such as functional and multivalued dependencies, that we assume about "legal" relations over that scheme, that cause redundancy. The appropriate response to redundancy is decomposition, the replacement of one relation scheme t
doi:10.1007/3-540-10843-2_13 fatcat:j3mq6xblgfa6faorgg6h2f53fm