Identifiers as primary keys
Tables store information for business entities using attributes of relevant data types. A row in the CUSTOMER table holds information for a given customer, and a row in the ORDERS table represents an order—or does it? Perhaps in this example, orders can contain multiple products and span just as many rows. To determine a unique instance of an entity, an identifier—or primary key (PK), if referring to a physical database—is used.
A PK is a column or set of columns whose values uniquely determine an instance of an entity. Only one PK can be defined per table. From a business perspective, a PK represents a single entity instance. To return to the previous example—is an order a single row containing one product, or does our organization allow multiple products per order? PKs inform database users of what that reality looks like at the table level.
The following figure shows some sample data from a fictitious ORDERS table.
