Database-Management-System
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Question 44
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Which of the following relational calculus expressions is not safe?
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{t|∃u ∈ R1 (t[A] = u[A])∧ ¬∃s ∈ R2 (t[A] = s[A])}
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{t|∀u ∈ R1 (u[A]= “x” ⇒ ∃s ∈ R2 (t[A] = s[A] ∧ s[A] = u[A]))}
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{t|¬(t ∈ R1)}
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{t|∃u ∈ R1 (t[A] = u[A])∧ ∃s ∈ R2 (t[A] = s[A])}
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Correct Answer: C