Question

Suppose that :

-A relation Movies(ID, name, type)
-ID is a key attribute and name, type are not key attributes
- t1, t2 are two instances of Movies
What relation algebra is used to express a key constraint on Movies?
Select one:
a. st1.ID¹t2.ID AND = AND = (t1 x t2)=Æ
b. t1.ID=t2.ID AND t1.name¹t2.name AND t1.type¹t2.type (t1 x t2)=Æ
c. st1.ID=t2.ID AND = AND = (t1 x t2)=Æ
d. st1.ID¹t2.ID AND t1.name¹t2.name AND t1.type¹t2.type (t1 x t2)=Æ

Answer 1

The correct relation algebra for expressing the key constraint in the Movies relation is the one that states there should be no two distinct tuples with the same ID in the Cartesian product of the Movies relation with itself.

Step-by-step explanation:

The relation algebra used to express a key constraint on a relation, such as the Movies relation given in this scenario, ensures that the attribute designated as the key is unique for each tuple in the relation. Considering the Movies relation with ID as the key attribute, the correct relation algebra expression that would enforce the key constraint is the one that makes sure there are no two different tuples with the same ID. This can be achieved by saying that there should be no pairs of distinct tuples (t1, t2) in the Cartesian product of the Movies relation with itself where the ID attributes in both tuples are the same.

The correct choice, therefore, would be:

t1.ID = t2.ID AND t1.name ≠ t2.name AND t1.type ≠ t2.type implies (t1 x t2) = ∅.

This represents that for any two tuples t1 and t2 in the relation, if they have the same ID, they must not appear together in the Cartesian product (representing that they cannot be distinct tuples), thus enforcing the uniqueness of the key attribute.