Question-9

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?

• Both S1 and S2 are true
• S1 is true and S2 is false
• S1 is false and S2 is true
• Both S1 and S2 are false

Answer

• Both S1 and S2 are true
• S1 is true and S2 is false
• S1 is false and S2 is true
• Both S1 and S2 are false

Solution

Relation Scheme	Foreign Keys
Employees	DepartmentID
Employees	ManagerID

In the above example, the Employees relation scheme contains two foreign keys: one referring to the DepartmentID in the Departments relation and another referring to the ManagerID within the same Employees relation.

The fact that there are two foreign keys contradicts S1 and the fact that the foreign key ManagerID refers to the same relation Employees contradicts S2

Hence, both S1 and S2 are false.