Logical vs. physical database design

Logical vs. Physical Database Design

Database design consists of two major stages: Logical Design and Physical Design. Each stage serves a different purpose in structuring data for efficiency and maintainability.

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1. Logical Database Design

Logical design focuses on what data should be stored and how data elements relate without considering the physical storage details.

Key Features

✔ Platform-independent: Doesn't depend on a specific database system (MySQL, PostgreSQL, etc.).
✔ Conceptual modeling: Uses Entity-Relationship (ER) models to define entities, attributes, and relationships.
✔ Normalization: Eliminates redundancy and ensures data integrity.

Example of Logical Design (ER Model)

Entities:
- Customer (id, name, email)
- Order (id, order_date, customer_id)

Relationships:
- One Customer can place many Orders (1:M).

Logical schema:

Customer (customer_id PK, name, email)  
Order (order_id PK, order_date, customer_id FK)  

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2. Physical Database Design

Physical design focuses on how data is stored and accessed in a specific database management system (DBMS).

Key Features

✔ Platform-dependent: Optimized for a specific DBMS (e.g., indexing in MySQL differs from PostgreSQL).
✔ Storage considerations: Defines file structures, indexing, and partitioning.
✔ Performance tuning: Optimizes queries, caching, and disk I/O.

Example of Physical Design (SQL Implementation)

Using MySQL:

CREATE TABLE Customer (
    customer_id INT PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(100) NOT NULL,
    email VARCHAR(100) UNIQUE NOT NULL
);

CREATE TABLE Order (
    order_id INT PRIMARY KEY AUTO_INCREMENT,
    order_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    customer_id INT,
    FOREIGN KEY (customer_id) REFERENCES Customer(customer_id) ON DELETE CASCADE
);

Additional physical optimizations:
- Indexes: CREATE INDEX idx_customer_email ON Customer(email);
- Partitioning: Splitting large tables across storage units.

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3. Key Differences

Feature	Logical Design	Physical Design
Focus	Data relationships & constraints	Storage, performance, indexing
Platform Dependence	DBMS-independent	DBMS-specific
Representation	ER Diagrams, Normalized Tables	SQL schema, tables, indexes
Optimization	Eliminates redundancy, ensures consistency	Query optimization, partitioning, caching
Implementation	Conceptual structure	Actual database creation

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4. When to Use Each

• Logical Design: Used during early development to define data structure & integrity.
• Physical Design: Used when implementing the database in a specific DBMS with performance considerations.