Topic
Universal relation assumption
About: Universal relation assumption is a research topic. Over the lifetime, 15 publications have been published within this topic receiving 1942 citations.
Papers
TL;DR: There are precise conditions that should be satisfied in a semantically meaningful extension of the usual relational operators, such as projection, selection, union, and join, from operators on relations to operators on tables with “null values” of various kinds allowed.
Abstract: This paper concerns the semantics of Codd's relational model of data. Formulated are precise conditions that should be satisfied in a semantically meaningful extension of the usual relational operators, such as projection, selection, union, and join, from operators on relations to operators on tables with “null values” of various kinds allowed. These conditions require that the system be safe in the sense that no incorrect conclusion is derivable by using a specified subset Ω of the relational operators; and that it be complete in the sense that all valid conclusions expressible by relational expressions using operators in Ω are in fact derivable in this system. Two such systems of practical interest are shown. The first, based on the usual Codd's null values, supports projection and selection. The second, based on many different (“marked”) null values or variables allowed to appear in a table, is shown to correctly support projection, positive selection (with no negation occurring in the selection condition), union, and renaming of attributes, which allows for processing arbitrary conjunctive queries. A very desirable property enjoyed by this system is that all relational operators on tables are performed in exactly the same way as in the case of the usual relations. A third system, mainly of theoretical interest, supporting projection, selection, union, join, and renaming, is also discussed. Under a so-called closed world assumption, it can also handle the operator of difference. It is based on a device called a conditional table and is crucial to the proof of the correctness of the second system. All systems considered allow for relational expressions containing arbitrarily many different relation symbols, and no form of the universal relation assumption is required. Categories and Subject Descriptors: H.2.3 [Database Management]: Languages— query languages; H.2.4 [Database Management]: Systems— query processing General Terms: Theory
1,194 citations
TL;DR: This work proposes a simpler method of describing the real world, where constraints are given by functional dependencies and a single join dependency, and characterize in terms of hypergraphs those multivalued dependencies that are the consequence of a given join dependency.
Abstract: One problem concerning the universal relation assumption is the inability of known methods to obtain a database scheme design in the general case, where the real-world constraints are given by a set of dependencies that includes embedded multivalued dependencies We propose a simpler method of describing the real world, where constraints are given by functional dependencies and a single join dependency The relationship between this method of defining the real world and the classical methods is exposed We characterize in terms of hypergraphs those multivalued dependencies that are the consequence of a given join dependency Also characterized in terms of hypergraphs are those join dependencies that are equivalent to a set of multivalued dependencies
317 citations
1 Jan 1980
TL;DR: System/U as mentioned in this paper is a universal relation database system under development at Standford University which uses the language C on UNIX and is intended to test the use of the universal view, in which the entire database is seen as one relation.
Abstract: System/U is a universal relation database system under development at Standford University which uses the language C on UNIX. The system is intended to test the use of the universal view, in which the entire database is seen as one relation. This paper describes the theory behind System/U, in particular the theory of maximal objects and the connection between a set of attributes. We also describe the implementation of the DDL (Data Description Language) and the DML (Data Manipulation Language), and discuss in detail how the DDL finds maximal objects and how the DML determines the connection between the attributes that appear in a query.
116 citations
TL;DR: The theory behind System/U is described, in particular the theory of maximal objects and the connection between a set of attributes and the implementation of the DDL and the DML.
Abstract: System/U is a universal relation database system under development at Standford University which uses the language C on UNIX. The system is intended to test the use of the universal view, in which the entire database is seen as one relation. This paper describes the theory behind System/U, in particular the theory of maximal objects and the connection between a set of attributes. We also describe the implementation of the DDL (Data Description Language) and the DML (Data Manipulation Language), and discuss in detail how the DDL finds maximal objects and how the DML determines the connection between the attributes that appear in a query.
113 citations
IBM1
TL;DR: Although central to the current direction of dependency theory, the assumption of a universal relation is incompatible with some aspects of relational database theory and practice.
Abstract: Although central to the current direction of dependency theory, the assumption of a universal relation is incompatible with some aspects of relational database theory and practice. Furthermore, the universal relation is itself ill defined in some important ways. And, under the universal relation assumption, the decomposition approach to database design becomes virtually indistinguishable from the synthetic approach.
100 citations
Related Topics (5)
Performance Metrics
| Year | Papers |
|---|---|
| 1986 | 1 |
| 1985 | 1 |
| 1984 | 3 |
| 1983 | 3 |
| 1982 | 3 |
| 1981 | 1 |