Primary key attributes appear in the top compartment. These provide the primary identification scheme for the entity type. These provide an alternate identification scheme. If an alternate key has two or more components, each component has the same value for n. Different alternate keys must have different values for n.
Attributes are mandatory by default.
All attributes are single-valued. In Figure 2, empNr is the primary key of Employee and the bldgNr, roomNr pair provides the composite primary key for Room. All attributes are mandatory except for fax and phoneNr, which are marked optional. Connection relationships are either identifying or non-identifying. In principle, an identifying relationship corresponds to an association that is conceptually used in the identification scheme of the child.