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

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Chapter 20. S

safe expression An expression that would be guaranteed to evaluate to a finite result even if the underlying domains were infinite. In practice, various rules are imposed to ensure that unsafe expressions can never occur. An example of an unsafe expression, if it were permitted, would be one denoting the set of all supplier tuples not currently appearing in relvar S (in other words, a request for the complement of the relation that's the current value of relvar S). Note: As should be obvious, it's generally desirable to prohibit unsafe expressions even if all domains are in fact finite (as of course they are in real systems).

scalar 1. (Of a type, attribute, value, or variable) Having no user-visible component parts. The term is also ...

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