In Sections 11.1, 11.2, and 11.3 we introduce the basic concepts of the Entity–Relationship model, namely entities, relationships, and attributes. In each section we illustrate how the basic ER concepts are represented pictorially in an ER diagram using UML. In Section 11.4 we differentiate between weak and strong entities and in Section 11.5 we discuss how attributes normally associated with entities can be assigned to relationships. In Section 11.6 we describe the structural constraints associated with relationships. Finally, in Section 11.7 we identify potential problems associated with the design of an ER model called connection traps and demonstrate how these problems can be resolved.