Physical Structure

A Class of data is identified by a common attributes set. LastName, FirstName, MiddleInitial, DateOfBirth and Gender are common attributes of the Class Person. Class data exists independent of its use. Class data are enterprise data. Class data are functionally independent and belong in the enterprise data layer.

During the analysis and design of the enterprise/application database pair, Class data is abstracted out of the tables in the existing physical data models or other documentation to the enterprise data layer. What remains in the application data layer are data that are dependent on a business function or activity, such as "Employee.GrossPay, Employee.FICA and Employee.NetPay." Entity data are application data. Entity data are functionally dependent and belong in the application data layer.

Relationships between Classes are always many-to-many. One Person may perform many Activities. One Activity may involve many Persons. The relationships between Classes and Entities should be one-to-one. Misspelled and/or duplicate data in legacy datasets may produce relationships that are one-to-many, but those are in error and should be resolved.