Types of DBMS Entities and their examples

Entity in DBMS can be a real-world object with an existence, For example, in a College database, the entities can be Professor, Students, Courses, etc.

Entities has attributes, which can be considered as properties describing it, for example, for Professor entity, the attributes are  Professor_Name, Professor_Address, Professor_Salary, etc. The attribute value gets stored in the database.

Example of Entity in DBMS

Let us see an example −

<Professor>

Professor_ID	Professor_Name	Professor_City	Professor_Salary
P01	Tom	Sydney	$7000
P02	David	Brisbane	$4500
P03	Mark	Perth	$5000

Here, Professor_Name, Professor _Address and Professor _Salary are attributes.
          Professor_ID is the primary key

Types of DBMS Entities

The following are the types of entities in DBMS −

Strong Entity

The strong entity has a primary key. Weak entities are dependent on strong entity. Its existence is not dependent on any other entity.

Strong Entity is represented by a single rectangle −

Continuing our previous example, Professor is a strong entity here, and the primary key is  Professor_ID.

Weak Entity

The weak entity in DBMS do not have a primary key and are dependent on the parent entity. It mainly depends on other entities.

Weak Entity is represented by double rectangle −

Continuing our previous example, Professor is a strong entity, and the primary key is  Professor_ID. However, another entity is Professor_Dependents, which is our Weak Entity.

<Professor_Dependents>

Name

DOB

Relation

This is a weak entity since its existence is dependent on another entity Professor, which we saw above. A Professor has Dependents.

Example of Strong and Weak Entity

The example of a strong and weak entity can be understood by the below figure. 

The Strong Entity is Professor, whereas Dependent is a Weak Entity. 

ID is the primary key (represented with a line) and the Name in Dependent entity is called Partial Key (represented with a dotted line).