ER model entity

Description:

• An entity is something that users want to track.
  • Examples include customers, purchases, products, etc.
• Entities of a given type are grouped into an entity class such as EMPLOYEE (a collection of all EMPLOYEE entities) which are shown in all caps.
• An entity instance of an entity class is the occurrence of a particular entity, such as CUSTOMER 12345.

Attributes:

• Entities have attributes, which describe the entity’s characteristics.
  • Examples include EmployeeName, DateOfHire and JobSkillCode.
• Attributes are shown with the first letter of each word capitalized.
• An attribute has a data type (character, numeric, date, currency, etc.)

Strong entity

Weak entity

Associative entity:

• An associative entity (also called an association entity) is used whenever a pure N:M relationship cannot properly hold attributes that are describing aspects of the relationship between two entities.
• A new entity is then created to:
  • link the two original entities
  • hold the attributes

Subtype entity

• A subtype entity is a special case of another entity called supertype.
• An attribute of the supertype indicates which of the subtypes is appropriate for a given instance and is called a discriminator.
• Subtypes can be exclusive or inclusive:
  • if exclusive, the supertype relates to at most one subtype
  • if inclusive, the supertype can relate to one or more subtypes
• The relationships that connect supertypes and subtypes are called IS- A relationship because a subtype is the same entity as the supertype.
• The identifier of a supertype and all of its subtypes is the same attribute