May 2013 Archive

Projecting Functional Dependencies

When we learn design of relation schemas, we shall also have need to answer the following question about FD's. Assume we have a relation R with some FD's F, and we "project" R by removing certain attributes from the schema. Consider S is the relation that results from R if we

Design of Relational Database Schemas / Anomalies

Careless selection of a relational database schema can lead to problems. For instance, Example (b) of "Combining Relations" showed what happens if we try to combine the relation for a many-many relationship with the relation for one of its entity sets. The major problem we

Decomposing Relations

The accepted way to remove these anomalies is to decompose relations. Decomposition of R involves splitting the attributes of R to make schemas of two new relations. Our decomposition rule also includes a way of populating those relations with tuples by "projecting" the tuples

Boyce-Codd Normal Form

The objective of decomposition is to replace a relation by several that do not exhibit anomalies. There is, it turns out, a simple condition under which the anomalies discussed above can be guaranteed not to exist. This condition is called Boyce-Codd

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