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

  • πB(r1) - πC(r2) = Ø
  • πC(r2) - πB(r1) = Ø
  • πB(r1) = πC(r2)

Answer is "pB(r1) - pC(r2) = Ø"

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