William would like to thank Mike Cross (mcross@racenter.com), data warehouse director at Rent-A-Center, for his contributions to this month's column.

Events and states are similar and related but need to be modeled and treated distinctly within your data warehouse. An event is simply a point-in-time occurrence and has only one time associated with it. This does not imply that an event can happen only once, rather, an event is a single occurrence. A state, on the other hand, represents something over a period of time and has a specific start and end time. States may have a null end time, but only one per subject. Examples of inventory events are delivery, return, to-service and from-service. Inventory states are on-rent, idle, on-loan and in-service. Events often trigger changes in states, e.g., a delivery begins the on-rent state, and if enough events are defined, maintaining the state of an inventory item is not necessary. Maintaining the state of events makes querying and reporting much easier and does not require the understanding of complicated business rules that associate events with states by the user. The business rules behind state changes can be implemented within the extract, transform and load (ETL) and with triggers, removing this burden from the report developers and user community.

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