Multiple choice

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?

  1. $\prod_{B}(r_{1})- \prod_{C}(r_{2})= \varnothing$
  2. $\prod_{C}(r_{2})- \prod_{B}(r_{1})= \varnothing$
  3. $\prod_{B}(r_{1}) = \prod_{C}(r_{2})$
  4. $\prod_{B}(r_{1}) - \prod_{C}(r_{2}) \neq \varnothing$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Since B is a foreign key referring C, $ \prod_{B}(r_{1}) \subseteq \prod_{C}(r_{2} \Rightarrow \prod_{B}(r_{1})- \prod_{C}(r_{2})= \varnothing$

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