An entity is the database equivalent of a noun. Distinguishable objects such as employees, order items, departments, and products are all examples of entities. In a database, a table represents each entity. The entities that you build into your database arise from the activities for which you will be using the database, such as tracking sales calls and maintaining employee information.
Each entity contains a number of attributes. Attributes are particular characteristics of the things that you would like to store. For example, in an employee entity, you might want to store an employee ID number, first and last names, an address, and other information that pertains to a particular employee. Attributes are also known as properties.
You depict an entity using a rectangular box. Inside, you list the attributes associated with that entity.
An identifier is one or more attributes on which all the other attributes depend. It uniquely identifies an item in the entity. Underline the names of attributes that you want to form part of an identifier.
In the Employee entity, above, the Employee Number uniquely identifies an employee. All the other attributes store information that pertains only to that one employee. For example, an employee number uniquely determines an employee's name and address. Two employees might have the same name or the same address, but you can make sure that they don't have the same employee number. Employee Number is underlined to show that it is an identifier.
It is good practice to create an identifier for each entity. As will be explained later, these identifiers become primary keys within your tables. Primary key values must be unique and cannot be NULL or undefined. They identify each row in a table uniquely and improve the performance of the database server.