Which of the tuple relational calculus expression is NOT safe?

(a)

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

• t[A] is restricted by values from R1

• Also checked against values in R2

• All variables are range-restricted

✅ Safe

(b)

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

• u ranges over R1

• s ranges over R2

• t[A] is indirectly bound through s[A]

✅ Safe

(c)

{ t | ¬( t ∈ R1 ) }

• t is not restricted to any relation

• It can take any possible tuple value

• Result may be infinite

❌ NOT SAFE

(d)

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

• t[A] is restricted by attributes of R1 and R2

• All variables are properly bound

✅ Safe