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Relational algebra
Relational algebra, first described by E.F. Codd while at IBM, is a family of algebra with a well-founded semantics used for modelling the data stored in relational databases, and defining queries on it.
The main application of relational algebra is providing a theoretical foundation for relational databases, particularly query languages for such databases, chief among which is SQL.
== Introduction ==
Relational algebra received little attention outside of pure mathematics until the publication of E.F. Codd's relational model of data in 1970. Codd proposed such an algebra as a basis for database query languages. (See section Implementations.)
Five primitive operators of Codd's algebra are the ''selection'', the ''projection'', the ''Cartesian product'' (also called the ''cross product'' or ''cross join''), the ''set union'', and the ''set difference''.
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