During the process of listing attributes for each user type (individual, evaluator, and auditor), the database designers notice that the attributes of the three user types are nearly identical and that all three user types can reside in one table: the Person table. The only adjustment they need to make to accommodate all types of users in one table is to allow a NULL value in the e-mail address field, because e-mail-related information is not required for persons such as auditors, who are not necessarily part of the organization. The following diagram illustrates how the three user types can reside in one table, the Person table, and how the user types (A for auditor, E for evaluator, and I for individual) are implemented.
The most important way in which user types in the Person table vary is in their relationship to groups (Group table). Constraints on Eval Table Columns describes how the PersonType column in the PersonGroup table identifies the roles of users in a group.