2 - The Entity–Relationship Model

Abstract

This chapter defines all the major entity–relationship (ER) concepts that can be applied to the conceptual data modeling phase of the database life cycle. An entity is a person, place, thing, or event of informational interest. Attributes are objects that provide descriptive information about entities. Attributes may be unique identifiers or non-unique descriptors. Relationships describe the connectivity between entity instances: one-to-one, one-to-many, or many-to-many. The degree of a relationship is the number of associated entities: two (binary), three (ternary), or any n (n-ary). The role (name), or relationship name, defines the function of an entity in a relationship. The concept of existence in a relationship determines whether an entity instance must exist (mandatory) or not (optional). The concept of generalization allows for the implementation of supertype and subtype abstractions. This simple form of ER models is used in most design tools and is easy to learn and apply to a variety of industrial and business applications. It is also a very useful tool for communicating with the end user about the conceptual model and for verifying the assumptions made in the modeling process. A more complex form, a superset of the simple form, is useful for the more experienced designer who wants to capture greater semantic detail in diagram form, while avoiding having to write long and tedious narrative to explain certain requirements and constraints.