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Database query results may contain repeated records, with their multiplicities retained.

A physical recipe for evaluating a database query, specifying scans, index accesses and implementations of relational joins and duplicate elimination.

The requirement that a database query have a finite, appropriately range-restricted answer on finite input. In the usual domain-independent interpretation, extending the ambient value domain with unused values must not change the answer. The absence of a negation symbol alone does not ensure safety.

A database query has the same result when its ambient domain is extended while all input database relations are unchanged.

Assume the two database queries have compatible output database attributes. ThenFor a finite input database, the result of the relational difference is a subset of the finite result of $Q_1$, so it is finite and introduces no new values. For the usual domain independence of a database query, let $D$ be the input database and let $A\subseteq A^{\prime }$ be two ambient value domains containing its data and the query constants. Database query safety givesand thereforeAlthough a domain relational calculus expression for the relational difference uses negation, its positive membership requirement in $Q_1$ supplies the necessary range restriction. Negation is not by itself a reason for failure of database query safety.

Database query results contain no repeated database tuples. SQL usually uses bag semantics unless duplicate elimination is requested.