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The Relational Database Dictionary, Extended Edition by C. J. Date

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Chapter 10. I

idempotence Let Op be a dyadic operator, and assume for definiteness that Op is expressed in infix style. Then Op is idempotent if and only if, for all x, x Op x = x.

Examples: In logic, OR and AND are both idempotent, because x OR x = x and x AND x = x for all x. It follows as a direct consequence that UNION and JOIN, respectively, are idempotent in relational algebra.

identity 1. (General) That which distinguishes a given entity from all others. 2. (Operator) Equality. 3. (Logic) Equality; also, a tautology of the form (p) EQUIV (q). 4. (Comparison) A Boolean expression of the form (exp1) = (exp2), where exp1 and exp2 are expressions of the same type, that's guaranteed to evaluate to TRUE regardless of the values of any variables ...

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