Identifying Entities and Attributes

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Based on the requirements that you collect, identify the objects that need to be in the database—the entities and attributes. An entity is a type of person, object, or thing that needs to be described in the database. It might be an object with a physical existence, like a person, a car, or an employee, or it might be an object with a conceptual existence, like a company, a job, or a project. Each entity has properties, called attributes, that describe it.

For example, suppose you are designing a database that must contain information about each employee in the company, departmental-level information, information about current projects, and information about customers and sales. The example below shows how to create a list of entities and attributes that organizes the required data.

List of entities and attributes

Entities	Attributes
EMPLOYEE	Employee Number
	Last Name
	First Name
	Department Number
	Job Code
	Phone Extension
	Salary
DEPARTMENT	Department Number
	Department Name
	Department Head Name
	Department Head Employee Number
	Budget
	Location
	Phone Number
PROJECT	Project ID
	Project Name
	Project Description
	Team Leader
	Product
CUSTOMER	Customer Number
	Customer Name
	Contact Name
	Phone Number
	Address
SALES	PO Number
	Customer Number
	Sales Rep
	Order Date
	Ship Date
	Order Status

By listing the entities and associated attributes this way, you can begin to eliminate redundant entries. Do the entities in your list work as tables? Should some columns be moved from one group to another? Does the same attribute appear in several entities? Each attribute should appear only once, and you need to determine which entity is the primary owner of the attribute.

For example, DEPARTMENT HEAD NAME should be eliminated because employee names (FIRST NAME and LAST NAME) already exist in the EMPLOYEE entity. DEPARTMENT HEAD EMPLOYEE NUM can then be used to access all of the employee-specific information by referencing EMPLOYEE NUMBER in the EMPLOYEE entity. For more information about accessing information by reference, see Establishing Relationships between Objects.

The next section describes how to map your lists to actual database objects—entities to tables and attributes to columns.

Advance To:

• Designing Tables