Which of the tuple relational calculus expression is NOT safe?

GATE · Computer Science · 2001

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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