Suppose R1(A, B) and R2(C, D) are two relation schemas. Let r1 and r2 be the corresponding relation instances. B is a foreign key that refers to C in R2. If data in r1 and r2 satisfy referential integrity constraints, which of the following is ALWAYS TRUE?
1
ΠB(r1) - ΠC(r2) = Φ
2
ΠC(r2) - ΠB(r1) = Φ
3
ΠB(r1) = ΠC(r2)
4
ΠB(r1) - ΠC(r2) ≠ Φ