The Entiy Relationship (ER) model consists of different types of entities. The existence of an entity may depends on the existence of one or more other entities, such an entity is said to be existence dependent.Entities whose existence not depending on any other entities is termed as not existence dependent.
Entities based on their characteristics are classified as follows.
- Strong Entities
- Weak Entities
- Recursive Entities
- Composite Entities
Strong Entity Vs Weak Entity
An entity set that has a primary key is termed as strong entity set. An entity set that does not have sufficient attributes to form a primary key is termed as a weak entity set.
A weak entity is existence dependent. That is the existence of a weak entity depends on the existence of an identifying entity set.
The discriminator (or partial key) is used to identify other attributes of a weak entity set.
The primary key of a weak entity set is formed by primary key of identifying entity set and the discriminator of weak entity set.
The existence of a weak entity is indicated by a double rectangle in the ER diagram.
We underline the discriminator of a weak entity set with a dashed line in the ER diagram.
A recursive entity is one in which a relation can exist between occurrences of the same entity set. This occurs in a unary relationship.