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Which of the following relational calculus expressions is not safe?
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- { t | ∃u ∈ R1(u[A]) ∧ ̚ ∃s ∈ R2 (t[A] = s[A]) }
- { t | ∀u ∈ R1(u[A]) = "x" ⇒ ∃s ∈ R2 (t[A] s[A] ∧ s[A] = u[A])) }
- { t | ̚(t ∈ R1) }
- { t | ∃u ∈ R1(t[A]) = u[A] = u[A] ∧ ∃s ∈ R2 (t[A] = s[A])) }
- { t | ∃u ∈ R1(u[A]) ∧ ̚ ∃s ∈ R2 (t[A] = s[A]) }
Correct Option: C
{t | ̚ (t ∈ R)}
This option contradicts itself. We want a tuple t and t itself is in r1 and we want other tuples than t by using negation so it’s not safe.
