---

ER Diagrams

• ERD stands for Entity Relationship diagram.
• It is a graphical representation of an information system.
• ER diagram shows the relationship between objects, places, people, events etc. within that system.
• It is a data modeling technique which helps in defining the business process.
• It used for solving the design problems.

Features of ER Model

• This model is widely used to develop the initial design of the database.
• This model provides a collection of basic concepts which can be easily implemented in a database system.
• It describes data as a set of entities, attributes, and relationships.
• It provides the graphical or conceptual view of the database design.

Uses of Entity-Relationship Model:

• ER models can be used in designing the database.
• It provides useful concepts that allow a user to move from an informal description to precise description.
• This model or diagram was developed to facilitate database design by allowing the specification of the overall conceptual structure of a database.
• It is widely useful in mapping the meanings and interactions of real world enterprises onto a conceptual schema.
• These models can be used for the logical or conceptual design of database applications.

Advantages of ER Model

• Entity-Relationship Diagram or model is easy to create and understand. 
• Entity-Relationship Model is very simple because if you know the relationship between attributes and entities, you can easily draw it.
• It is a tool for effective communication for database designer.
• It can be easily converted into a Relational Model by simply converting the ER Model into tables.

Limitations/Disadvantages of ER Diagram

Disadvantages of an Entity-Relationship Diagram include the following aspects:

• Entity-Relationship diagram is not suitable for those systems which contain semi-structured or unstructured data set.
• In the ER model, it is difficult to show data manipulation.

This model is especially popular for high-level design.

Components of ER Diagram

An ER Diagram consists of the following components:

1. Entity
2. Attributes
3. Relationships

Here we will discuss about Entity and Attributes. Next we will discuss about Relationships.

Following are the components of ER Diagram,

Notations	Representation	Description
	Rectangle	It represents the Entity.
	Ellipse	It represents the Attribute.
	Diamond	It represents the Relationship.
	Line	It represents the link between attribute and entity set to relationship set.
	Double Rectangle	It represents the weak entity.
	Composite Attribute	It represents composite attribute which can be divided into subparts. For eg. Name can be divided into First Name and Last Name
	Multi valued Attribute	It represents multi valued attribute which can have many values for a particular entity. For eg. Mobile Number.
	Derived Attribute	It represents the derived attribute which can be derived from the value of related attribute.
	Key Attribute	It represents key attribute of an entity which have a unique value in a table. For eg. Employee ? EmpId (Employee Id is Unique).

Entity:

An entity may be any object, class, person or place. In the ER diagram, an entity can be represented as rectangles.

Consider an organization as an example- manager, product, employee, department etc. can be taken as an entity.

Weak Entity

An entity that depends on another entity called a weak entity. The weak entity doesn't contain any key attribute of its own. The weak entity is represented by a double rectangle.

Attribute

The attribute is used to describe the property of an entity. Eclipse is used to represent an attribute.

For example, id, age, contact number, name, etc. can be attributes of a student.

Key Attribute

The key attribute is used to represent the main characteristics of an entity. It represents a primary key. The key attribute is represented by an ellipse with the text underlined.

Composite Attribute

An attribute that composed of many other attributes is known as a composite attribute. The composite attribute is represented by an ellipse, and those ellipses are connected with an ellipse.

Multivalued Attribute

An attribute can have more than one value. These attributes are known as a multivalued attribute. The double oval is used to represent multivalued attribute.

For example, a student can have more than one phone number.

Derived Attribute

An attribute that can be derived from other attribute is known as a derived attribute. It can be represented by a dashed ellipse.

For example, A person's age changes over time and can be derived from another attribute like Date of birth.

Importance of ER Model

• ER Model is plain and simple for designing the structure.
• It saves time.
• Without ER diagrams you cannot make a database structure & write production code.
• It displays the clear picture of the database structure.

---