In my last column, I explored the major purpose of the logical (business) data model. This issue, I'll explore the objective of the system and technology models. The system model is a logical portrayal of the data needed for an application, while the technology model is the physical representation. Their main purposes are to support construction and maintenance by ensuring an understanding of the business rules, providing a basis for a physical data structure that meets performance objectives and enables coordination among multiple applications.

While the business data model portrays the enterprise view of the business rules, the system model provides an understanding of the business rules within the realm of the individual application's needs. Often, individual applications address only part of the organization, and hence the model to be used for the application needs to be adjusted for both scope and business rules. Let's consider how this applies to an application being built to handle only commercial customers for a firm that has other types of customers, such as consumers and industrial customers. The scope adjustment removes entities (e.g., household) and attributes (e.g., consumer subtype attributes) that do not apply to commercial customers; the business rule adjustment impacts relationships (some may not apply), as well as optionalities and cardinalities of relationships that remain. The rules depicted in the system model are transferred into the physical model, with retention of mandatory foreign key relationships and accommodation for nullable foreign keys for tables with optional parents.

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