**calculus** 1. Generically, a system of formal computation (the Latin word *calculus* means a pebble, perhaps used in counting or some other form of reckoning). 2. Relational calculus specifically (if the context demands).

**candidate key** Loosely, a unique identifier. More precisely, let *K* be a subset of the heading of relvar *R*; then *K* is a candidate key (key for short) for *R* if and only if (a) no possible value for *R* contains two distinct tuples with the same value for *K* (the uniqueness property), while (b) the same can't be said for any proper subset of *K* (the irreducibility property). Note that every relvar, base or derived, has at least one key. Note too that, by definition, keys are sets of attributes (and key values are therefore tuples); ...

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