Entities And Attributes

An entity is something of interest to the user about which to collect or store data. It is also called a data aggregate because it represents a number data element. In our sales status system, the "sales" entity contains data elements such as the salesperson's number, name, and date of employment, and the sales period covered by the report. The "item" entity has data elements such as item number, item description, and the sale price of each item.

 Data entities are explained by the use of several terms: attribute, value key and instance of an entity. For example, a salesperson (entity) is described by attributes such as number, name, sex, age, and height. So attributes describe an entity. They are physically stored in fields or data elements.

Each attribute takes on a unique value. For example, "211306801" is a unique value of the attribute "salesperson number." An attribute, takes, on a value for-a specific occurrence (or instance) of an entity. The value is Jim old. Other values that describe him are male, 34 years old, and 5 feet 10 inches tall. So the instance of the entity is a 34-year-old, 5 feet 10 inches tall, male salesman with social security number 211306801.

A key is a unique identifier of the entity. In our example, the key 211306801 is a unique identifier of Jim Arnold. Sex, age, and height are no identifiers, since they are not unique. They are monkey identifiers.