System-assigned keys
A system-assigned primary key is usually preferable to a composite key. A system-assigned key is a number or code that is attached to each instance of an entity when the entity is first entered into the database. The easiest system-assigned keys to implement are serial numbers because the database server can generate them automatically. IBM® Informix® database servers offer the SERIAL, SERIAL8, and BIGSERIAL data types for serial numbers. However, the people who use the database might not like a plain numeric code. Other codes can be based on actual data; for example, an employee identification code can be based on a person's initials combined with the digits of the date that they were hired. In the telephone directory example, a system-assigned primary key is used for the name table.