ER Model: The Foundation of SQL Databases

(ER components)

The Entity-Relationship Model (ER Model) is a conceptual model for designing a databases. This model represents the logical structure of a database, including entities, their attributes and relationships between them.

• Entity: An objects that is stored as data such as Student, Course or Company.

• Attribute: Properties that describes an entity. For example: StudentID, CourseName, or EmployeeEmail.

• Relationship: A connection between entities such as "a Student enrolls in a Course".

The graphical representation of this model is called an Entity-Relation Diagram (ERD).

Why Use ER Diagrams In DBMS?

• ER diagrams represent the E-R model in a database, making them easy to convert into relations (tables).

• Unlike technical schemas, ER diagrams require no technical knowledge of the underlying DBMS used.

Symbols Used in ER Model

What is an Entity?

An Entity represents a real-world object, concept or thing about which data is stored in a database. It act as a building block of a database. Tables in relational database represent these entities.

Example of entities:

1. Real-World Objects: Person, Car, Employee etc.

2. Concepts: Course, Event, Reservation etc.

3. Things: Product, Document, Device etc.

What is an Entity Set?

An entity refers to an individual object of an entity type, and the collection of all entities of a particular type is called an entity set. For example, E1 is an entity that belongs to the entity type "Student," and the group of all students forms the entity set.

Types of Entity

There are two main types of entities:

1. Strong Entity

It is a type of entity that has a key Attribute that can uniquely identify each instance of the entity. A Strong Entity does not depend on any other Entity in the Schema for its identification. It has a primary key that ensures its uniqueness and is represented by a rectangle in an ER diagram.

2. Weak Entity

A Weak Entity cannot be uniquely identified by its own attributes alone. It depends on a strong entity to be identified. A weak entity is associated with an identifying entity (strong entity), which helps in its identification. A weak entity are represented by a double rectangle. The participation of weak entity types is always total. The relationship between the weak entity type and its identifying strong entity type is called identifying relationship and it is represented by a double diamond.

Example:

A company may store the information of dependents (Parents, Children, Spouse) of an Employee. But the dependents can't exist without the employee. So dependent will be a Weak Entity Type and Employee will be identifying entity type for dependent, which means it is Strong Entity Type.

Attributes in ER Model

For example, for a Student entity Roll_No, Name, DOB, Age, Address, and Mobile_No are the attributes that define entity type Student. In ER diagram, the attribute is represented by an oval.

Types of Attributes

1. Key Attribute

The attribute which uniquely identifies each entity in the entity set is called the key attribute. For example, Roll_No will be unique for each student. In ER diagram, the key attribute is represented by an oval with an underline.

2. Composite Attribute

An attribute composed of many other attributes is called a composite attribute. For example, the Address attribute of the student Entity type consists of Street, City, State, and Country. In ER diagram, the composite attribute is represented by an oval comprising of ovals.

3. Multivalued Attribute

An attribute consisting of more than one value for a given entity. For example, Phone_No (can be more than one for a given student). In ER diagram, a multivalued attribute is represented by a double oval.

4. Derived Attribute

An attribute that can be derived from other attributes of the entity type is known as a derived attribute. e.g.; Age (can be derived from DOB). In ER diagram, the derived attribute is represented by a dashed oval.

The Complete Entity Type Student with its Attributes can be represented as:

(Entity and Attributes)

Relationship Type and Relationship Set

A Relationship Type represents the association between entity types. For example, ‘Enrolled in’ is a relationship type that exists between entity type Student and Course. In ER diagram, the relationship type is represented by a diamond and connecting the entities with lines.

(Entity-Relationship Set)

A set of relationships of the same type is known as a relationship set. The following relationship set depicts S1 as enrolled in C2, S2 as enrolled in C1, and S3 as registered in C3.

(Relationship Set)

Degree of a Relationship Set

The number of different entity sets participating in a relationship set is called the degree of a relationship set.

1. Unary/Recursive Relationship: When there is only ONE entity set participating in a relation, the relationship is called a unary relationship. For example, one person is married to only one person.

2. Binary Relationship: When there are TWO entities set participating in a relationship, the relationship is called a binary relationship. For example, a Student is enrolled in a Course.

3. Ternary Relationship: When there are three entity sets participating in a relationship, the relationship is called a ternary relationship.

4. N-ary Relationship: When there are n entities set participating in a relationship, the relationship is called an n-ary relationship.

Cardinality in ER Model

The maximum number of times an entity of an entity set participates in a relationship set is known as cardinality.

Cardinality can be of different types:

1. One-to-One

When each entity in each entity set can take part only once in the relationship, the cardinality is one-to-one.

Example: Person and Passport

Person: Each person can have only one passport.

Passport: Each passport belongs to only one person.

2. One-to-Many

In one-to-many mapping as well where each entity can be related to more than one entity. Let us assume that one surgeon department can accommodate many doctors. So the Cardinality will be 1 to M. It means one department has many Doctors.

3. Many-to-One

When entities in one entity set can take part only once in the relationship set and entities in other entity sets can take part more than once in the relationship set, cardinality is many to one.

Let us assume that a student can take only one course but one course can be taken by many students. So the cardinality will be n to 1. It means that for one course there can be n students but for one student, there will be only one course.

4. Many-to-Many

When entities in all entity sets can take part more than once in the relationship cardinality is many to many. Let us assume that a student can take more than one course and one course can be taken by many students. So the relationship will be many to many.

How to Draw an ER Diagram

1. Identify Entities: The very first step is to identify all the Entities. Represent these entities in a Rectangle and label them accordingly.

2. Identify Relationships: The next step is to identify the relationship between them and represent them accordingly using the Diamond shape. Ensure that relationships are not directly connected to each other.

3. Add Attributes: Attach attributes to the entities by using ovals. Each entity can have multiple attributes (such as name, age, etc.), which are connected to the respective entity.

4. Define Primary Keys: Assign primary keys to each entity. These are unique identifiers that help distinguish each instance of the entity. Represent them with underlined attributes.

5. Remove Redundancies: Review the diagram and eliminate unnecessary or repetitive entities and relationships.

6. Review for Clarity: Review the diagram make sure it is clear and effectively conveys the relationships between the entities.