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 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 Name in Dependent entity is called Partial Key (represented with a dotted line).

raja
Published on 02-Jul-2018 17:09:48
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