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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 operations of the relational algebra are the set operations (such as union, intersection, and cartesian product), selection (keeping only some lines of a table) and the projection (keeping only some columns).
The relational data model describes how the data is structured.
Codd's reduction algorithm can convert from relational calculus to relational algebra.