There is a distinction between the schema of a relation and the extension of a relation. The extension of a relation may vary a lot over time, but the schema for a relation is usually quite stable. In conceptual terms, a relation is a structure for storing values of the attributes of a particular class of entities, or a particular class of relationships. The example is a relation describing members of the entity class Customers. Relations can also describe relationships.
Compound keys are common in relationships.
For example, if we were creating a relation to represent students enrolled in classes we would need the key for that relation to be the key for students combined with the key for classes. 3.3 Simple Example of Accounts Receivable Modeled Using the Relational Model Entity Relationship for this example. City and State can be determined if Zip Code is known, thus a transitive dependency exists. Customer ID identifies the specific customer and serves also to identify that customer's rows in the Invoices and Payments relations. A customer may have many invoices but an invoice will have exactly one customer.