What is different between DFD and ERD?

Data Flow Diagrams (DFD) and Entity-Relationship Diagrams (ERD) are both essential tools in the fields of systems analysis and database design. However, they serve different purposes, focus on distinct aspects of a system, and utilize different symbols and notations. Understanding the differences between DFDs and ERDs is crucial for effectively designing and analyzing information systems and databases.

1. Purpose and Focus

• Data Flow Diagram (DFD):

  • Purpose: DFDs are used to represent the flow of data within a system. They focus on how data moves between processes, data stores, and external entities.
  • Focus: The primary focus is on the processes that transform data, the data flows between these processes, and the sources and destinations of the data.

• Entity-Relationship Diagram (ERD):

  • Purpose: ERDs are used to model the logical structure of a database. They represent the entities within the system and the relationships between them.
  • Focus: The primary focus is on data entities, their attributes, and the relationships that define how entities interact with each other.

2. Key Components

• Data Flow Diagram (DFD):

  • Processes: Represented by circles or rounded rectangles, processes transform incoming data flows into outgoing data flows.
  • Data Stores: Represented by open-ended rectangles or parallel lines, data stores indicate where data is stored within the system.
  • Data Flows: Represented by arrows, data flows show the direction of data movement between processes, data stores, and external entities.
  • External Entities: Represented by squares or rectangles, external entities are sources or destinations of data outside the system being modeled.

• Entity-Relationship Diagram (ERD):

  • Entities: Represented by rectangles, entities are objects or concepts with distinct existence (e.g., Customer, Order).
  • Attributes: Represented by ovals or listed within entities, attributes are properties or details about entities (e.g., CustomerID, OrderDate).
  • Relationships: Represented by diamonds or lines connecting entities, relationships define how entities interact or are associated with each other (e.g., Places, Contains).
  • Cardinality: Indicates the numerical relationships between entities (e.g., one-to-one, one-to-many, many-to-many).

3. Use Cases

• Data Flow Diagram (DFD):

  • System Analysis: Used to analyze and design the flow of data within existing or proposed systems.
  • Process Improvement: Helps identify inefficiencies, redundancies, or bottlenecks in data processing.
  • Requirement Gathering: Assists in understanding how data is handled and processed, aiding in the collection of system requirements.

• Entity-Relationship Diagram (ERD):

  • Database Design: Serves as a blueprint for designing the structure of a database.
  • Data Modeling: Helps in visualizing data entities and their interrelationships, ensuring that all necessary data is captured and properly linked.
  • Documentation: Provides a clear and organized representation of the database schema for developers, analysts, and stakeholders.

4. Level of Abstraction

• Data Flow Diagram (DFD):

  • Process-Oriented: Focuses on the movement and transformation of data through processes.
  • Higher-Level View: Can represent different levels of detail (e.g., Level 0 for a high-level overview, Level 1 for more detailed processes).

• Entity-Relationship Diagram (ERD):

  • Data-Oriented: Focuses on data entities and their relationships, independent of how data is processed.
  • Detailed Structure: Provides a detailed view of the data model, including entities, attributes, and relationships.

5. Notations and Symbols

• Data Flow Diagram (DFD):

  • Gane-Sarson Notation: Uses circles for processes, arrows for data flows, open-ended rectangles for data stores, and squares for external entities.
  • Yourdon Notation: Uses rectangles for processes, arrows for data flows, and parallelograms for data stores and external entities.

• Entity-Relationship Diagram (ERD):

  • Chen Notation: Uses diamonds for relationships, rectangles for entities, and ovals for attributes.
  • Crow's Foot Notation: Uses lines with crow's foot symbols to denote cardinality, rectangles for entities, and ovals for attributes.
  • Crow’s Foot Extended Notation: Includes additional symbols for attributes and keys.

6. Examples

Data Flow Diagram (DFD) Example:

Imagine an online bookstore system. A Level 1 DFD might include:

• Processes:
  • 1.0 Process Order
  • 2.0 Manage Inventory
• Data Stores:
  • Data Store 1: Customer Database
  • Data Store 2: Order Database
• External Entities:
  • External Entity: Customer
  • External Entity: Payment Gateway
• Data Flows:
  • Customer sends order details to Process Order.
  • Process Order retrieves customer information from Customer Database.
  • Process Order sends order confirmation to Order Database and Payment Gateway.

Entity-Relationship Diagram (ERD) Example:

For the same online bookstore system, an ERD might include:

• Entities:
  • Customer
  • Order
  • Product
  • Category
• Attributes:
  • Customer: CustomerID (PK), Name, Email
  • Order: OrderID (PK), OrderDate, TotalAmount, CustomerID (FK)
  • Product: ProductID (PK), ProductName, Price, CategoryID (FK)
  • Category: CategoryID (PK), CategoryName
• Relationships:
  • Customer places Order (One-to-Many)
  • Order contains Product (Many-to-Many, typically handled via an associative entity like OrderDetails)
  • Product belongs to Category (Many-to-One)

7. Summary of Differences

When to Use Each Diagram

• Use a DFD when:

  • You need to understand or communicate how data moves through a system.
  • Analyzing existing processes to identify inefficiencies or areas for improvement.
  • Designing new systems by mapping out data flow requirements.

• Use an ERD when:

  • Designing or modeling the database structure.
  • Defining the data entities, their attributes, and relationships for a new or existing database.
  • Ensuring data integrity and consistency within the database design.

Conclusion

While both DFDs and ERDs are vital in the realm of systems analysis and database design, they serve distinct purposes. DFDs are process-oriented and focus on how data flows within a system, making them ideal for analyzing and designing system processes. ERDs, on the other hand, are data-oriented and concentrate on the structure and relationships of data within a database, making them essential for database design and modeling. Utilizing both diagrams effectively can lead to a comprehensive understanding and robust design of information systems and databases.