Relational Algebra

A relational algebra operates on relations, as it is relational algebra is a procedural query language based on a small number of operators. It acts as an intermediate language used by a RDBMS. By applying special operators to relations, queries are made.

There are six fundamental operations select, project, rename, Cartesian product, union, set-difference. Out of these six select, project, rename are unary and rest are binary, i.e., cartesian product, union, set- difference.

Several other operations are also there which are defined in terms of the fundamental operations, viz., set-intersection, natural join, division, assignment.

A basic expression consists of either
• A relation in the database.
• A constant relation.

General expressions are formed out of smaller sub-expressions using
• σ_q(E₁) select (q a predicate)
• П_z(E₁) project (z a list of attributes)
• ρ_x(E₁) rename (x a relation name)
• E₁ U E₂ union
• E₁ - E₂ set difference
• E₁ * E₂ cartesian product