Definitions and Business Rules

Business rules are a critical part of the data model. Business rules take the form of relationships, rolenames, candidate keys, defaults, and other modeling structures. Modeling structures include generalization categories, referential integrity, and cardinality. Business rules are also captured in entity and attribute definitions and validation rules.

For example, a CURRENCY entity can be defined as follows:

The set of all valid currencies recognized anywhere in the world, or a subset of these that our company has decided to use in its day to day business operations.

The entity definition contains a subtle, but important distinction. In the latter case, there is a business rule, or policy statement, involved. This rule manifests itself in the validation rules for �currency code.� This rule restricts the valid values for �currency code� to the values used by the business. Maintenance of the business rule becomes a task of maintaining the table of valid values for CURRENCY. To permit or prohibit trading of CURRENCYs, you simply create or delete instances in the table of valid values.

The attributes �bought currency code� and �sold currency code� are similarly restricted. However, both are further restricted using a validation rule that says �bought currency code� and �sold currency code� cannot be equal. Therefore, each is dependent on the value of the other in its actual use. Validation rules can be addressed in the definitions of attributes, and can also be defined explicitly using validation rules, default values, and valid value lists.