Weak Entity in DBMS

In this article you will know about Weak Entity in DBMS

What are entities?

An entity is an object of concern used to represent the things in the real world, e.g., car, table, book, etc. .
An entity need not be a physical entity, it can also represent a concept in real world, e.g., project, loan, etc…
It represents a class of things, not any one instance, e.g., ‘STUDENT’ entity has instances of ‘Ramesh’ and ‘Mohan’.

Weak Entity in DBMS

Entity types that do not contain any key attribute, and hence cannot be identified independently, are called weak entity types. A weak entity can be identified uniquely only by considering some of its attributes in conjunction with the primary key attributes of another entity, which is called the identifying owner entity.

Generally a partial key is attached to a weak entity type that is used for unique identification of weak entities related to a particular owner entity type. The following restrictions must hold:

The owner entity set and the weak entity set must participate in one to many relationship set. This relationship set is called the identifying relationship set of the weak entity set.
The weak entity set must have total participation in the identifying relationship.

EXAMPLE: Consider the entity type dependent related to employee entity, which is used to keep track of the dependents of each employee. The attributes of dependents are: name, birth date, sex and relationship. Each employee entity is said to own the dependent entities that are related to it. However, please note that the ‘dependent’ entity does not exist of its own, it is dependent on the Employee entity.

In other words we can say that in case an employee leaves the organisation all dependents related to him/her also leave along with. Thus, the “dependent entity has no significance without the entity employee’. Thus, it is a weak entity. The notation used for weak entities is explained in Section 2.6. Weak Entity in DBMS