Entity integrity

An entity is any person, place, or thing to be recorded in a database. Each table represents an entity, and each row of a table represents an instance of that entity. For example, if order is an entity, the orders table represents the idea of an order and each row in the table represents a specific order.

To identify each row in a table, the table must have a primary key. The primary key is a unique value that identifies each row. This requirement is called the entity integrity constraint.

For example, the orders table primary key is order_num. The order_num column holds a unique system-generated order number for each row in the table. To access a row of data in the orders table, use the following SELECT statement:

SELECT * FROM orders WHERE order_num = 1001;

Using the order number in the WHERE clause of this statement enables you to access a row easily because the order number uniquely identifies that row. If the table allowed duplicate order numbers, it would be almost impossible to access one single row because all other columns of this table allow duplicate values.

For more information on primary keys and entity integrity, see the Informix® Database Design and Implementation Guide.