Past exam of the computer science course of the University of Oxford / 2015 / Databases / 1 / c / i / Solution

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.