Relational database: Encyclopedia II - Relational database - Relational algebra

Relational database - Relational algebra

The relational algebra is a set of operations that manipulate relations as they are defined in the relational model and as such describes part of the data manipulation aspect of this data model. Because of their algebraic properties these operations are often used in database query optimization as an intermediate representation of a query to which certain rewrite rules can be applied to obtain a more efficient version of the query.

The exact set of operations may differ per definition and also depends on whether the unlabeled relational model (that uses mathematical relations) or the labeled relational model (that uses the labeled specialization of mathematical relations) is used. We will assume the labeled case here as this is the most common way the relational model is defined. That means that we assume that tuples are partial functions from attribute names to values. The value of the tuple t on the attribute a is denoted in this article as t(a).

Adapted from the Wikipedia article "Relational algebra", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki