Question

Say that some attribute X1 in relation R2 is a foreign key referring to A1, an attribute in relation R1. What must be true of this relation?

Answer 1

In a relation, a foreign key is a reference to an attribute in another relation. Certain conditions must be true for this relationship to hold.

Step-by-step explanation:

In a relation, if attribute X1 is a foreign key referring to attribute A1 in another relation, it means that X1 in the first relation is a reference to A1 in the second relation. This establishes a relationship between the two relations where X1 depends on A1. In order for this relationship to hold, certain conditions must be true:

1. The attribute A1 in the second relation should be a primary key or a unique key.
2. The values in X1 should exist as values in A1 in the second relation.
3. The referential integrity constraint should be enforced, which means that any changes made to A1 in the second relation should be reflected in X1 in the first relation.

For example, if we have two relations 'Customers' and 'Orders' and attribute 'customer_id' in the 'Orders' relation is a foreign key referencing 'customer_id' in the 'Customers' relation, the above conditions should be satisfied to establish a valid relationship between them.