ER Diagram in DBMS
Updated on 18th Jan, 24 9.1K Views

In this blog, we will get insight into the ER diagram, its significance along with the components and symbols used in it. For a better understanding, we will look into the construction of ER diagrams and their application.

Table of Contents

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What is an ER Diagram?

A relational database management system (RDBMS) is a type of database management system that organizes data into tables, which consist of rows and columns. In the context of an RDBMS, an entity-relationship (ER) diagram is a graphical representation used for database design to illustrate the relationships between tables and the structure of the database. Tables in an RDBMS represent entities, and relationships between tables indicate how these entities are related. Entities contain attributes (characteristics or properties), and relationships define how these entities are related and interact with each other.

ER diagrams help to visually conceptualize and organize the data model, providing a clear and concise way to communicate the structure of a database system. They are an essential tool in the initial stages of database development, helping in the design and understanding of the relationships and constraints within the data.

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Why Do We Need ER Diagrams?

Entity-relationship (ER) diagrams are a crucial tool in database design and development for several reasons:

  • ER diagrams give the database schema a visual representation, which simplifies comprehension of the database structure for stakeholders, developers, and designers.
  • ER diagrams help in the design process by assisting in the identification and definition of entities, attributes, and relationships in accordance with the system requirements.
  • ER diagrams are a useful tool for communicating with various stakeholders, such as database administrators, business analysts, and developers.
  • ER diagrams provide a visual representation of the entities and relationships that need to be modeled in the database, which helps in gathering and analyzing requirements.
  • ER diagrams help with the normalization process, which is essential for removing redundancy and enhancing database performance.
  • By defining primary keys, foreign keys, and relationships and making sure the database complies with established guidelines, ER diagrams support maintaining data integrity.
  • ER diagrams help developers plan and optimize database queries by illuminating the relationships between entities.

Symbols of an ER Diagram

Symbols of an ER diagram

Rectangle: 

  • Representation: Entity
  • Usage: In a relational database, this represents a table that includes the entity’s name.

Ellipse

  • Representation: Attribute 
  • Usage: Used to illustrate a feature or attribute of an object. Features are listed inside the entity rectangle.

Double Ellipse

  • Representation: Multivalued Attribute
  • Usage: Represents an attribute for which an entity can have more than one value. It’s symbolized by two ellipses.

Diamond

  • Representation: Relationship
  • Usage: Depicts the relationship that exists between two or more entities. The relationship’s name is contained in the diamond form.

Lines (Cardinality)

  • Representation: Cardinality
  • Usage: Shows how many instances of a particular entity are possible to link to another entity. One-to-one (1:1), one-to-many (1:N), and many-to-many (N: N) cardinalities are common.

Double Rectangle

  • Representation: Weak Entity
  • Usage: Represents an entity that is not able to be identified exclusively by its characteristics. Its identity is dependent upon an additional entity (the owner entity).

Components of an ER Diagram

In an entity-relationship (ER) diagram, various symbols are used to represent different components of the data model. These symbols collectively form a visual representation of the database structure, facilitating communication between stakeholders involved in the design and development of a database system.

Components of an ER Diagram
  1. Entity: A rectangle is used to represent a unique real-world object, idea, or entity that can be uniquely identified and defined in a database. Typically, the rectangle’s interior contains the entity name.
Entity in ER Diagram

a. Weak Entity: Represented by a double rectangle, a weak entity is an entity that does not have a primary key attribute of its own and depends on another entity (owner entity) for identification. For example: A parent is viewed as a strong entity and a child as a weak entity in a parent-child relationship.

Weak Entity in ER Diagram
  1. Attribute: An oval or ellipse is used to symbolize an attribute, which is a property or feature of an entity. A line connects them to their respective entities. The data that can be kept about an entity is described by its attributes.
Attribute in ER Diagram

a. Key Attribute: It is shown as an ellipse with an underlined attribute. Attributes that are part of the primary key or a unique key constraint are known as key attributes. 

Key Attribute in ER Diagram

b. Composite Attribute: Shown as a double ellipse, it denotes an attribute that has multiple sub-parts, each having a distinct meaning.

 Composite Attribute in ER Diagram

c. Derived Attribute: An attribute whose value can be obtained from other database attributes instead of being explicitly saved is called a derived attribute, and it is symbolized by a dashed oval.

Derived Attribute in ER Diagram

d. Multivalued Attribute: This type of attribute is indicated by a double oval that is joined to an entity by a double line. It can have more than one value for a single entity.

Multivalued Attribute in ER Diagram
  1. Relationship: A relationship shows how two entities are related to one another and is represented by a diamond shape. The relationship’s cardinality and participation restrictions are shown by the lines joining the entities to the diamond.
Relationship in ER Diagram

a. One to One: Each entity in the relationship is associated with at most one instance of the other entity, and vice versa.

For example, one employee has only one unique employee ID that belongs only to him.

One to One in ER diagram

b. One to Many: Each entity on the “one” side can be associated with multiple instances on the “many” side, but each entity on the “many” side is associated with at most one instance on the “one” side.

For example, a university has many students, but each student is enrolled in only one university.

One to Many in ER Diagram

c. Many to One: The reverse of “One-to-Many” – each entity in the “many” side can be associated with at most one instance on the “one” side, but each entity in the “one” side can be associated with multiple instances on the “many” side.

For example, many employees work in one organization, but each organization is managed by only one employee.

Many to One in ER Diagram

d. Many to Many: Each entity in the relationship can be associated with multiple instances of the other entity, and vice versa.

For example, a bookstore sells many books, and each book can be sold by many bookstores.

Many to Many in ER Diagram

Participation Constraint for an ER Diagram 

The minimum and maximum participation of entities in a relationship are defined by participation constraints in an entity-relationship diagram. They indicate whether involvement in a relationship is required of an entity or not. Partial and total participation are the two categories of participation restrictions.

Total Participation: This means that all of the entities in the entity set have to take part in the relationship. It is usually shown as a solid line joining the relationship line to the entity.

Example:  In a university database, every course must have at least one instructor and every instructor must be assigned to at least one course.

Partial Participation: This allows entities to be optional participants in a relationship. It means that an entity may or may not be associated with the relationship. It is denoted by an open circle or a dashed line connecting the entity to the relationship line.

Example: In a library database, some books may be checked out by patrons, but not all books need to be checked out.

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How to Create an ER Diagram?

Creating an ER diagram involves multiple steps. Here’s a step-by-step guide on how to draw an ER diagram.

Step 1: Start by identifying the key entities in your system. For each entity, list its attributes.

Step 2: The second step is to determine the relationships between entities. Relationships represent how entities are connected in the real world.

Step 3: Represent each entity with a rectangle or ellipse shape. Inside the shape, write the entity name.

Step 4: Within the entity shape, list the attributes, separating them with commas or colons.

Step 5: Represent relationships with lines connecting the corresponding entity shapes. For many-to-many relationships, use a diamond shape at the intersection of the lines.

Step 6: Label the lines with the relationship type and cardinality. Cardinality indicates the number of entities that can participate in the relationship.

Step 7: Once the basic diagram is complete, refine it by adding additional symbols and annotations as needed. Review the diagram for accuracy and consistency.

Here is an ER diagram for the marks database.

ER Diagram for marks database

Applications of ER Diagrams

In database design and modeling, entity-relationship (ER) diagrams are essential tools. They are employed to graphically depict the relationships, entities, and attributes that make up a database’s structure. The following are some important uses for ER diagrams:

  • Database Structure: The creation of an ER diagram is essential to database schema design. They serve as a guide for building an orderly and effective database by assisting in the identification of entities, their characteristics, and the connections among them.
  • Communication with Stakeholders: Database designers and other stakeholders, such as clients, end users, and other team members, can communicate with each other using ER diagrams. Compared to technical documentation, they offer a visual representation that is simpler to comprehend.
  • Analyzing requirements: ER diagrams help in comprehending and evaluating the system requirements in the early phases of a project. They help in determining the constraints, relationships, and data entities that the database design must take into account.
  • Normalization: ER diagrams help with the normalization process, which is necessary to increase a database’s efficiency. In order to reduce redundancy and enhance data integrity, normalization entails organizing data, and ER diagrams are useful for determining the dependencies between entities.
  • Query Enhancement: Database queries can be optimized for better performance by developers by knowing the relationships between entities. ER diagrams facilitate effective data retrieval and manipulation by illuminating the connections between various entities.
  • Data Integration: In scenarios where multiple databases or systems need to share and integrate data, ER diagrams help in understanding the relationships between different entities and support the integration process.

Conclusion

In conclusion, entity-relationship diagrams in database management systems serve as powerful tools for visualizing the relationships, attributes, and tables within a database. They provide a concise representation of the data model, assisting in efficient database design, normalization, and communication with stakeholders. ER diagrams play a pivotal role in enhancing data organization, optimizing queries, and facilitating seamless integration, making them indispensable in the realm of database development and management.

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