By Paul Beynon-Davies (auth.)
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Retrieval in the relational data model is conducted using a series of operators known collectively as the relational algebra. 1 Relational Algebra The relational algebra is a set of eight operators. Each operator takes one or more relations as input and produces one relation as output. The three main operators of the algebra are restrict, project and join. Using these three operators, most of the manipulation required of relational systems can be accomplished. The additional operators - product, union, intersection, difference and division - are modelled on the traditional operators of set theory.
The tables below confonn to all of these rules and hence constitute relations: Modules moduleName level courseCode staffNo 1 1 CSD CSD CSD CSD CSD 234 234 345 345 237 Relational Database Systems Relational Database Design Deductive Databases Object-Oriented Databases Distributed Databases 3 3 2 Lecturers staffNo staffName status 234 237 345 Davies T Jones S Evans R L SL PL Database Systems 25 Codd originally borrowed from the terminology of mathematics to denote elements of his data model. Columns of tables are known as attributes.
A statement in the calculus can include logical connectives (and, or, not) and the quantifiers (the existential quantifier, Forsome, and the universal quantifier Forall). Forsome, sometimes read as there exists, has the meaning that in a given set of tuples there is at least one tuple which satisfies a given condition. status INTORI WHERE Forsome I (1. status - SL) has exactly the same result as the first statement. Forall has the meaning that in a given set of tuples exactly all tuples satisfy a given condition.