GATE CSE 2021 Set 2 | Question: 6

Question

Consider the following statements $S1$ and $S2$ about the relational data model:

• $S1$: A relation scheme can have at most one foreign key.
• $S2$: A foreign key in a relation scheme $R$ cannot be used to refer to tuples of $R.$

Which one of the following choices is correct?

• A. Both $S1$ and $S2$ are true
• B. $S1$ is true and $S2$ is false
• C. $S1$ is false and $S2$ is true
• D. Both $S1$ and $S2$ are false

Comments

What if the foreign key is NULL? Can we still be able to refer the tuples?

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Why they have to call them tuples and create confusion. Why can't they say it as attributes.

Answer 1

Both S1 and S2 are false.

S1 is false because you can have more than 1 FK in one relation for example if u see the case of many to many realtion with both the entities are paritally participating in that case you form one extra table for relationship in which you have PK of both the entity relatiuon as FK so u are having more than one FK in one relation which is possible.

S2 is false because you can have self refrential FK also for example take case of self refrential relationship with both partial participation you will get 1 FK in the table itself refrening the PK of the table.

Answer 2

Option D is correct:

S1 is false we can have more than one foreign key as per requirement
S2 is false  foregin key act upon columns so we can use it to refer to a tuple.