## ER diagram-

• ER diagram or Entity Relationship diagram is a conceptual model that gives the graphical representation of the logical structure of the database.
• It shows all the constraints and relationships that exist among the different components.

## Components of ER diagram-

An ER diagram is mainly composed of following three components-

1. Entity Sets
2. Attributes
3. Relationship Set

## Example-

Consider the following Student table-

 Roll_no Name Age 1 Akshay 20 2 Rahul 19 3 Pooja 20 4 Aarti 19

This complete table is referred to as “Student Entity Set” and each row represents an “entity”.

## Representation as ER Diagram-

The above table may be represented as ER diagram as-

Here,

• Roll_no is a primary key that can identify each entity uniquely.
• Thus, by using student’s roll number, a student can be identified uniquely.

## ER Diagram Symbols-

An ER diagram is composed of several components and each component in ER diagram is represented using a specific symbol.

ER diagram symbols are discussed below-

## 1. For Entity Sets-

An entity set is a set of same type of entities.

An entity refers to any object having-

• Either a physical existence such as a particular person, office, house or car.
• Or a conceptual existence such as a school or a company.

An entity set may be of the following two types-

1. Strong entity set
2. Weak entity set

### 1. Strong Entity Set-

• A strong entity set possess its own primary key.
• It is represented using a single rectangle.

### 2. Weak Entity Set-

• A weak entity set do not possess its own primary key.
• It is represented using a double rectangle.

## 2. For Relationship Sets-

• Relationship defines an association among several entities.
• A relationship set is a set of same type of relationships.

A relationship set may be of the following two types-

1. Strong relationship set
2. Weak relationship set

### 1. Strong Relationship Set-

• A strong relationship exists between two strong entity sets.
• It is represented using a diamond symbol.

### 2. Weak Relationship Set-

• A weak or identifying relationship exists between the strong and weak entity set.
• It is represented using a double diamond symbol.

## 3. For Attributes-

• Attributes are the properties which describes the entities of an entity set.
• There are several types of attributes.

## 4. For Participation Constraints-

Participation constraint defines the least number of relationship instances in which an entity has to necessarily participate.

There are two types of participation constraints-

1. Partial participation
2. Total participation

### 1. Partial Participation-

Partial participation is represented using a single line between the entity set and relationship set.

### 2. Total Participation-

Total participation is represented using a double line between the entity set and relationship set.

## 5. For Specialization and Generalization-

• Generalization is a process of forming a generalized super class by extracting the common characteristics from two or more classes.
• Specialization is a reverse process of generalization where a super class is divided into sub classes by assigning the specific characteristics of sub classes to them.

## 6. For Cardinality Constraints / Ratios-

Cardinality constraint defines the maximum number of relationship instances in which an entity can participate.

There are 4 types of cardinality ratios-

1. Many-to-many cardinality (m:n)
2. Many-to-one cardinality (m:1)
3. One-to-many cardinality (1:n)
4. One-to-one cardinality (1:1)