Suppose R1 (A, B) and R2 (C, D) are two relation schemas.

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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. Π_B(r₁) - Π_B(r₂) = ∅
2. Π_C(r₂) - Π_B(r₁) = ∅
3. Π_B(r₁) - Π_B(r₂)
4. Π_B(r₁) - Π_B(r₂) ≠ ∅

Correct Option: A

B is a foreign key in r₁ that refers to C in r₂ . r₁ and r₂ satisfy referential integrity constraints. So every value that exists in column B of r₁ must also exist in column C of r₂ .