Question

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 relational algebra expression that defines a key constraint on a Movies relation is option b, which stipulates that no two tuples can have the same ID but different names or types, ensuring uniqueness.

Step-by-step explanation:

The relational algebra expression used to express a key constraint on a Movies relation would ensure that each movie in the relation has a unique identifier. In relational algebra, a key constraint is expressed by stating that there are no two different tuples that have the same value for the key attribute. Considering the options, the correct expression would be:

b. t1.ID=t2.ID AND t1.name¹t2.name AND t1.type¹t2.type (t1 x t2)=∅

This expression states that there is no pair of different tuples (t1, t2) in the Cartesian product of the Movies relation with itself (denoted as t1 x t2) that have the same ID but different names or types. The symbol ∅ represents the empty set, which means that no such pair exists, thus enforcing the key constraint.