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Types of Entities

19 October, 2015 - 15:42

Independent entities, also referred to as Kernels, are the backbone of the database. It is 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

Example: Customer table, Employee table, Product table
Dependent Entities, also referred to as derived, depend on other tables for their meaning.

  • Dependent entities are used to connect two kernels together.
  • They are said to be existent 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:
  • use a composite of foreign keys of associated tables if unique
  • use a composite of foreign keys and qualifying column
  • create a new simple primary key

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

  • They represent multi-valued 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:
  • foreign key plus a qualifying column
  • or create a new simple primary key
  • Employee(EID, Name, Address, Age, Salary)
  • EmployeePhone(EID, Phone)