**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 ...

Start Free Trial

No credit card required