Question

What relational algebra is used to express a constraint that the attribute A of relation R refers to the attribute B of relation S?

Select one:
a. pA (R) = pB(S)
b. pA (R) Í pB(S)
c.pA (R) ∩ pB(S)=0
d. pA (R) ∪ pB(S)=0

Answer 1

The relational algebra for expressing that attribute A refers to attribute B is πA (R) ⊂ πB(S), indicating A's values are a subset of B's values within their respective relations.

Step-by-step explanation:

The relational algebra used to express a constraint that the attribute A of relation R refers to the attribute B of relation S is represented as πA (R) ⊂ πB(S). This notation means that the projection of attribute A from relation R is a subset of the projection of attribute B from relation S. In other words, every value of attribute A in relation R must also exist as a value of attribute B in relation S. This aligns with referential integrity constraints commonly used in database systems to ensure that relationships between tables maintain consistency.