relational algebra
<database, theory> 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}.
(1997-02-17)