Determining and defining business rules is the fifth phase of the database-design process. During this phase, you'll hold interviews, identify limitations on various aspects of the database, establish business rules, and define and implement validation tables.
The manner in which your organization views and uses its data will determine a set of limitations and requirements that you must build into the database. Your interviews with users and management will help you identify the specific constraints you will impose on the data, data structures, or relationships. You then establish and document these specifications as business rules.
The interviews you conduct with users will reveal specific limitations on various aspects of the database. For example, a user working with an order processing database is very aware of specific details, such as the fact that a ship date must occur later than an order date; that there must always be a daytime phone number; and that a shipping method should always be indicated. On the other hand, your interviews with management reveal general limitations on various aspects of the database. The office manager for an entertainment agency, for example, is familiar with general issues, such as the fact that an agent can represent no more than 20 entertainers and that promotional information for each entertainer must be updated every year.
Next, you define and implement validation tables as necessary to support certain business rules. For example, if you find that certain fields have a finite range of values because of the manner in which your organization uses them, you can use validation tables to ensure the consistency and validity of the values stored in those fields.
The level of integrity that business rules establish at this point is significant because it relates directly to the way your organization views and uses its data. As the organization grows, its perspective on the data will change, which means that the business rules must change as well. Determining and establishing business rules is an ongoing, iterative process, and you must be constantly diligent if you are going to maintain this level of integrity properly.