Entity - Relationship Model (E-R Model)

➺ E-R Model developed by Peter Chen in 1971 and it represents the conceptual view of a database thus it is called the conceptual data model.
➺ E-R model stands for Entity-Relationship Model. It is a high-level data model which describes the relationship among entity sets.
➺ In the E-R model, The database design is represented in the entity relationship diagram.

ER Diagram is a diagram that shows the relationships of an entity set in a database. It consists of Entities, Attributes, and relationships.

➺ Entity: It is an object which has physical existence and real-world object. The entity is represented by a rectangle.
➺ At the time of Database design, we give the name of the entity to the table name of the database.

➺ Entity Set: It refers to the collection of entities with attributes.

➺ Weak Entity: A weak entity refers entity that does not have its attribute. It is reliable on other attributes of the entity. This entity is represented by a double rectangle.

➺ A relationship is an association among two or more entities.

Types of relationships :-

○ One-to-One Relationships

➺When one entity is associated with one single entity and vice versa

○ One-to-Many Relationships

➺When one entity is associated with more than one entity.

○ Many-to-One Relationships

➺ When more than one entity is associated with one single entity.

○ Many-to-Many Relationships

➺ When more than one entity is associated with more than one entity and vice versa.

➺ Characterised properties of entity.

Types of Attributes:-

○ Simple Attribute

➺ An attribute that cannot divide into components further. It is also known as Atomic values For example:- Student's mobile number that cannot divide into components.

○ Composite Attribute

➺ An attribute composed of multiple other attributes is called a composite attribute.

○ Derived Attribute

➺ An attribute whose values are derived from other attributes of an entity is called a Derived attribute.

○ Multivalued Attribute

➺ An attribute that has more than one value is known as a multivalued attribute.

➺ Rectangles:- This symbol represent entity types.

➺ Ellipses:- This symbol represents attributes.

➺ Diamonds:- This symbol represents relationship types.

➺ Lines:- This symbol connects attributes to entity types with other relationship types.
➺ Double Ellipses:- This symbol represents multi-valued attributes.

➺ Primary key:- This symbol underlines the attributes.
➺ Double rectangles:- This symbol represents a weak entity.

➺ Double lines:- This symbol represents the total participation of the entity.
➺ Dashed Ellipses:- This symbol represents a derived attribute.

➺ Ellipses with lines:- This symbol represents a key attribute.

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