selection

In database theory, relational algebra is a theory that uses algebraic structures for modeling data and defining queries on it with well founded semantics. The theory was introduced by Edgar F. Codd as part of his relational model of data in 1970.

Overview

In relational algebra, a selection (sometimes called a restriction in reference to E.F. Codd's 1970 paper and not, contrary to a popular belief, to avoid confusion with SQL's use of SELECT, since Codd's article predates the existence of SQL) is a unary operation that denotes a subset of a relation. A selection is written as or where: and are attribute names is a binary operation in the set is a value constant is a relation The selection denotes all tuples in for which holds between the and the attribute. The selection denotes all tuples in for which holds between the attribute and the value .

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Also known as restriction