Q43

MCQ2 marks, −0.67

Suppose $R_{1} (\underline{A}, B)$ and $R_{2} (\underline{C}, D)$ are two relation schemas. Let $r_{1}$ and $r_{2}$ be the corresponding relation instances. $B$ is a foreign key that refers to $C$ in $R_{2}$ . If data in $r_{1}$ and $r_{2}$ satisfy referential integrity constraints, which of the following is ALWAYS TRUE?

(A)

$\prod_{B}(r_{1})- \prod _{C}(r_{2})= \varnothing$

(B)

$\prod_{C}(r_{2})- \prod _{B}(r_{1})= \varnothing$

(C)

$\prod_{B}(r_{1}) = \prod _{C}(r_{2})$

(D)

$\prod_{B}(r_{1}) - \prod _{C}(r_{2}) \neq \varnothing$

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