Which of the following relational calculus expressions is not safe?

GATE · 2001 · CS · Question 2 subparts

Which of the following relational calculus expressions is not safe?

1. A.

   {t | ∃u ∈ R1 (t[A] = u[A]) ∧ ¬∃s ∈ R2 (t[A] = s[A])}

2. B.

   {t | ∀u ∈ R1 (u[A] = "x" ⇒ ∃s ∈ R2 (t[A] = s[A] ∧ s[A] = u[A]))}

3. C.

   {t | ¬(t ∈ R1)}

4. D.

   {t | ∃u ∈ R1 (t[A] = u[A]) ∧ ∃s ∈ R2 (t[A] = s[A])}

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