Composition

January 11, 2010


New term for this month, borrowed from Algebra of Binary Relations: composition operation. Composition of two binary relations can be interpreted in Codd’s Relational Algebra (aka Algebra of Relations with Named Attributes) by taking their join, leading to a ternary relation, followed by a projection that removes the middle component. The less concise synonym for it is set intersection join. Renaming operation can be simulated as composition of the input relation with binary identity relation, which should convince the reader that composition is as frequent and important as natural join.

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