# CITS1402 Relational Database Management Systems Video 20 — Relational Algebra II.pdf

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CITS1402 Relational Database Management Systems Video 20 — Relational Algebra II Gordon Royle Department of Mathematics & Statistics Relational Algebra A legal expression in relational algebra is built from I Variables representing relations I Set-theoretic operators I “Relational” operators composed according to the rules of relational algebra. Every legal expression in relational algebra determines a relation . Operators I Set-theoretic operators The union of two relations R S , the intersection of two relations R S and the difference of two relations R - S . I Relational operators The projection operator π , the selection operator σ and the join operator ./ . Set Theory Operators Relational algebra permits the use of the standard set operations: I Union ( ) If R and S are union-compatible , then R S is the set of tuples in either R or S . I Intersection ( ) If R and S are union-compatible, then R S is the set of tuples in both R and S .