Multiple choice

Which of the following relationship calculus-expressions is not safe?

  1. $\left\\\{t \mid \exists u \in R_1\left(t[A] = u[A]\right) \land \neg \exists s \in R_2 \left(t[A] = s[A]\right)\right\\\}$
  2. $\left\\\{t \mid \neg (t \in R_1)\right\\\} $
  3. $\left\\\{t \mid \exists u \in R_1\left(t[A]=u[A]\right) \land \exists s \in R_2 \left(t[A] = s[A]\right)\right\\\}$

  4. Both (1) and (3) are safe.
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D Correct answer

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