Hierarchical schemata for relational databases

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Abstract
Most database design methods for the relational model produce a flat database, that is, a family of relations with no explicit interrelational connections. The user of a flat database is likely to be unaware of certain interrelational semantics. In contrast, the entity-relationship model provides schema graphs as a description of the database, as well as for navigating the database. Nevertheless, the user of an entity-relationship database may still commit semantic errors, such as performing a lossy join. This paper proposes a nonflat, or hierarchical, view of relational databases. Relations are grouped together to form relation hierarchies in which lossless joins are explicitly shown whereas lossy joins are excluded. Relation hierarchies resemble the schema graphs in the entity-relationship model. An approach to the design of relation hierarchies is outlined in the context of data dependencies and relational decomposition. The approach consists of two steps; each is described as an algorithm. Algorithm DEC decomposes a given universal relation according to a given set of data dependencies and produces a set of nondecomposable relation schemes. This algorithm differs from its predecessors in that it produces no redundant relation schemes. Algorithm RH further structures the relation schemes produced by Algorithm DEC into a hierarchical schema. These algorithms can be useful software tools for database designers.

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