BUS611: Data Management

You should also be familiar with different kinds of entities including independent entities, dependent entities and characteristic entities. These are described below.

Independent entities

Independent entities, also referred to as kernels, are the backbone of the database. They are what other tables are based on. Kernels have the following characteristics:

• They are the building blocks of a database.
• The primary key may be simple or composite.
• The primary key is not a foreign key.
• They do not depend on another entity for their existence.

If we refer back to our COMPANY database, examples of an independent entity include the Customer table, Employee table or Product table.

Dependent entities

Dependent entities, also referred to as derived entities, depend on other tables for their meaning. These entities have the following characteristics:

• Dependent entities are used to connect two kernels together.
• They are said to be existence dependent on two or more tables.
• Many to many relationships become associative tables with at least two foreign keys.
• They may contain other attributes.
• The foreign key identifies each associated table.
• There are three options for the primary key:
  1. Use a composite of foreign keys of associated tables if unique
  2. Use a composite of foreign keys and a qualifying column
  3. Create a new simple primary key

Characteristic entities

Characteristic entities provide more information about another table. These entities have the following characteristics:

• They represent multivalued attributes.
• They describe other entities.
• They typically have a one to many relationship.
• The foreign key is used to further identify the characterized table.
• Options for primary key are as follows:
  1. Use a composite of foreign key plus a qualifying column
  2. Create a new simple primary key. In the COMPANY database, these might include:
     • Employee (EID, Name, Address, Age, Salary) – EID is the simple primary key.
     • EmployeePhone (EID, Phone) – EID is part of a composite primary key. Here, EID is also a foreign key.