Entities and Attributes Defined

An entity is any person, place, thing, event, or concept about which information is kept. More precisely, an entity is a set or collection of like individual objects known as instances. An instance (row) is a single occurrence of a given entity. Each instance must have an identity distinct from all other instances.

In the preceding illustration, the CUSTOMER entity represents the set of all the possible customers of a business. Each instance of the CUSTOMER entity is a customer. You can list information for an entity in a sample instance table, such as is shown in the following illustration:

CUSTOMER

customer id	customer name	customer address
10001	Ed Green	Princeton, NJ
10011	Margaret Henley	New Brunswick, NJ
10012	Tomas Perez	Berkeley, CA
17886	Jonathon Walters	New York, NY
10034	Greg Smith	Princeton, NJ

Each instance represents a set of facts about the related entity. In the preceding table, each instance of the CUSTOMER entity includes information about the �customer id,� �customer name,� and �customer address.� In a logical model, these properties are known as the attributes of an entity. Each attribute captures a single piece of information about the entity.

You can include attributes in an ERD to describe the entities in the model more fully, as shown in the following illustration: