Database Concepts › Representing Relationships as Sets › Networks › Bill-of-Materials Structures

Bill-of-Materials Structures

A bill-of-materials structure is a network relationship between record occurrences of the same type. The name comes from the relationship among parts in an industrial assembly operation, where a part may be a component of another part and itself be comprised of other parts as its components.

At Commonweather, we find a bill-of-materials relationship among employees: an employee may report to a manager and also manage other people. The following diagram illustrates this relationship. Bill Ball manages June Moon and Peter Plum; Spiro Tutuo manages John Done and Sandy Shore. Both Bill Ball and Spiro Tutuo report to Frank O'Fill.

The standard technique for representing a bill-of-materials relationship is to use multiple membership and a junction record. Normally such a structure involves three record types and two sets, but in a bill-of-materials structure, two of the record types are the same.

The following diagram illustrates the bill-of-materials relationship among employees at Commonweather Corporation. A junction record, called STRUCTURE is used to relate employees and their managers. By finding the members of an occurrence of the REPORTS-TO set, and then finding their respective owners in the MANAGES set, we identify the managers of an employee. (If Commonweather uses matrix management, there might be multiple members in an occurrence of the REPORTS-TO set.) Reversing the process allows us to determine the employees that report to a given manager.