Different Types of Entities in DBMS

An entity is an object that exists and is distinguishable from other objects. In other words, the entity can be uniquely identified. There three different type of entities available in Database Management System.

1. Strong entity
2. weak entity
3. Associative entity

Depending upon how the entity exit, three types are entities can be drawn in an ER model.

1. Strong Entity

In an ER model, If an entity exist without depending other entities such entity is called strong entity. That means the existence of  an entity does not depend on other entities. The strong entity is represented using rectangle. For Example

2. Weak Entity

Weak entity is one whose existence depends on other entity. In many cases weak entity does not have primary key.

In this example, course is considered as weak entity because, if there are no students to take a particular course, then that course cannot be offered. The COURSE entity depends on the STUDENT entity.

3. Associative Entity or Bridge Entity

Basically it is a relation acts as an entity type that associates one or more entity types and contains attributes.

Associative entities are represented over relations. In the above example, enrollment is an associative entity, which acts as bridge between the entities students, course and the relation tech.