Natural keys created by operational source systems are subject to business rules outside the control of the DW/BI system. For instance, an employee number (natural key) may be changed if the employee resigns and then is rehired. When the data warehouse wants to have a single key for that employee, a new durable key must be created that is persistent and does not change in this situation. This key is sometimes referred to as a durable supernatural key. The best durable keys have a format that is independent of the original business process and thus should be simple integers assigned in sequence beginning with 1. While multiple surrogate keys may be associated with an employee over time as their proﬁle changes, the durable key never changes.