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)