Introduction to Database Management Systems

Introduction to Database Management Systems (DBMS)

Database Management Systems (DBMS) are an essential component of modern software applications and are integral to managing, storing, and retrieving data efficiently. They provide a systematic way to handle databases, ensuring data consistency, reliability, and security. This article covers the foundational concepts, components, and features of DBMS, including data abstraction, schemas, database languages, transaction management, architecture, and key database elements.

---

Purpose of a DBMS

The primary purpose of a DBMS is to provide a reliable, efficient, and user-friendly system to store and retrieve data. It acts as an intermediary between users and the data they interact with, ensuring the complexity of data storage is hidden while offering robust functionalities for data management.

Key objectives of a DBMS include:

• Efficient Data Storage and Retrieval: Utilizing sophisticated data structures to optimize data operations.
• Data Consistency and Integrity: Enforcing rules to maintain data correctness.
• Data Security: Restricting unauthorized access to sensitive data.
• Concurrency Control: Ensuring multiple users can access data simultaneously without conflicts.

---

Data Abstraction

Data abstraction simplifies how users interact with the database by hiding the complexity of data storage. It is divided into three levels:

1. Physical Level

• The lowest level of abstraction, describing how data is physically stored in the system.
• Focuses on low-level details like data blocks, file structures, and storage paths.
• Typically managed by database administrators and system developers.

2. Logical Level

• Describes what data is stored and the relationships between them.
• Provides a structured view of the entire database using tables, columns, and relationships.
• Facilitates physical data independence, allowing changes at the physical level without affecting the logical structure.

3. View Level

• The highest level of abstraction, offering a tailored perspective of the database for different users.
• Focused on simplifying interactions for end-users by hiding unnecessary complexity.
• A database may have multiple views catering to specific user needs.

---

Instances and Schemas

A database is defined in terms of its schema and instances.

• Schema:

  • The logical structure of the database, defining tables, relationships, and constraints.
  • Acts as a blueprint, remaining constant over time unless explicitly modified.

• Instance:

  • The data content stored in the database at a particular moment.
  • Continuously changes as data is inserted, updated, or deleted.

---

Database Languages

DBMSs use specialized languages to interact with databases. These are broadly classified into:

1. Data Definition Language (DDL)

• Defines the database structure and schema.
• Examples of operations:
  • CREATE: Define new tables or databases.
  • ALTER: Modify existing structures.
  • DROP: Remove tables or databases.
• Integrity Constraints in DDL ensure data accuracy and consistency:
  • Domain Constraints: Define permissible values for attributes.
  • Referential Integrity: Enforces valid relationships between tables.

2. Data Manipulation Language (DML)

• Enables users to manipulate data stored in the database.
• Common operations:
  • SELECT: Retrieve data.
  • INSERT: Add new data.
  • DELETE: Remove existing data.
  • UPDATE: Modify existing data.
• SQL provides a standardized DML syntax widely used across relational databases.

---

Transaction Management

A transaction is a logical unit of database operations that must adhere to the ACID properties to ensure reliability:

1. Atomicity: Transactions are indivisible; either all operations succeed, or none.
2. Consistency: Transactions must leave the database in a valid state.
3. Isolation: Concurrent transactions should not interfere with each other.
4. Durability: Once committed, changes persist even in case of system failures.

DBMS employs mechanisms such as locking, logging, and concurrency control to manage transactions and ensure these properties.

---

Database and Application Architecture

Modern databases follow the three-tier architecture to separate concerns and enhance scalability:

1. Presentation Tier:

   • User-facing layer, typically the front-end application.
   • Interacts with users through graphical interfaces or web pages.

2. Application Tier:

   • The logic layer where business rules and application logic are implemented.
   • Connects the front end with the database.

3. Database Tier:

   • The backend where data is stored and managed.
   • Includes the DBMS and the physical storage systems.

---

Database Users and Administrators

Types of Users:

1. End-Users: Interact with the database using applications or queries.
2. Application Programmers: Develop software applications using APIs provided by the DBMS.
3. Database Administrators (DBAs): Manage the database, control access, and ensure optimal performance.

Role of a DBA:

• Define and maintain schemas.
• Implement security measures.
• Monitor and optimize database performance.
• Perform backups and recovery operations.

---

Tables and Their Components

A table is the fundamental structure of a relational database, consisting of rows and columns.

• Rows (Tuples): Represent individual records in the table.
• Columns (Attributes): Represent data fields with specific data types.
• Primary Key: Uniquely identifies each row in the table.
• Foreign Key: Creates relationships between tables by referencing primary keys in other tables.

---

Keys in a Database

Keys are crucial in ensuring data integrity and establishing relationships. Common types include:

1. Primary Key: A unique identifier for table rows. Cannot contain NULL values.
2. Foreign Key: References a primary key in another table, enforcing referential integrity.
3. Candidate Key: Any column or set of columns that can uniquely identify a row. One candidate key is selected as the primary key.
4. Composite Key: A primary key consisting of two or more attributes.
5. Unique Key: Similar to a primary key but allows one NULL value.
6. Super Key: A superset of a candidate key that uniquely identifies rows.

---

Functions, Procedures, and Triggers in DBMS

In addition to managing and querying data, modern DBMSs provide mechanisms to encapsulate logic and automate tasks through functions, procedures, and triggers. These elements enhance the efficiency, maintainability, and responsiveness of database systems.

---

Functions

A function is a database object that performs a specific task and returns a single value. Functions are commonly used for calculations, data transformations, or retrieving specific information. They are similar to mathematical functions and can be invoked directly in SQL queries.

Characteristics of Functions:

• Input Parameters: Functions may accept zero or more input parameters.
• Return Value: A function always returns a single value of a specified data type.
• Read-Only: Functions cannot modify database tables or data directly; they are restricted to read-only operations.

Syntax for Creating Functions (SQL Example):

CREATE FUNCTION function_name (parameter_list)
RETURNS return_type
AS
BEGIN
    -- Function logic
    RETURN value;
END;

Example:

A function to calculate the total price of an order based on quantity and price per unit:

CREATE FUNCTION calculate_total_price(quantity INT, price_per_unit DECIMAL)
RETURNS DECIMAL
AS
BEGIN
    RETURN quantity * price_per_unit;
END;

Advantages:

1. Reusability of logic across queries.
2. Improved query readability and maintainability.
3. Enhanced performance by encapsulating complex logic.

---

Procedures

A procedure is a stored program in the database that performs a sequence of operations. Unlike functions, procedures do not return a value but can perform data modification tasks like INSERT, UPDATE, and DELETE.

Characteristics of Procedures:

• Can have input, output, and input-output parameters.
• Capable of modifying database tables.
• Executed using the CALL or EXEC statement.

Syntax for Creating Procedures (SQL Example):

CREATE PROCEDURE procedure_name (parameter_list)
AS
BEGIN
    -- Procedure logic
END;

Example:

A procedure to update the salary of an employee:

CREATE PROCEDURE update_salary(employee_id INT, new_salary DECIMAL)
AS
BEGIN
    UPDATE employees
    SET salary = new_salary
    WHERE id = employee_id;
END;

Advantages:

1. Encapsulation of complex logic into reusable units.
2. Ability to execute multiple operations in a single call.
3. Improved database performance by reducing network overhead.

---

Triggers

A trigger is a database object that automatically executes a predefined action in response to specific events on a table, such as INSERT, UPDATE, or DELETE operations.

Characteristics of Triggers:

• Defined on a specific table and activated by events.
• Can be fired before or after the event occurs.
• Used for enforcing business rules, maintaining audit logs, or propagating changes.

Types of Triggers:

1. BEFORE Trigger: Executes before the specified event.
2. AFTER Trigger: Executes after the specified event.
3. INSTEAD OF Trigger: Executes instead of the event (commonly used in views).

Syntax for Creating Triggers (SQL Example):

CREATE FUNCTION function_name (parameter_list)
RETURNS return_type
AS
BEGIN
    -- Function logic
    RETURN value;
END;

Example:

A trigger to log every new employee added to the employees table:

CREATE FUNCTION calculate_total_price(quantity INT, price_per_unit DECIMAL)
RETURNS DECIMAL
AS
BEGIN
    RETURN quantity * price_per_unit;
END;

Advantages:

1. Automatic enforcement of rules and policies.
2. Reduction in manual intervention for repetitive tasks.
3. Enhanced auditability by maintaining logs of changes.

---

Functions vs. Procedures vs. Triggers: Key Differences

Feature

Function

Procedure

Trigger

Feature	Function	Procedure	Trigger
Returns Value	Yes	No	No
Modifies Data	No	Yes	Yes
Execution	Invoked explicitly	Invoked explicitly	Invoked automatically
Use Case	Data computation	Complex operations	Event-driven actions

Returns Value

Yes

No

No

Modifies Data

No

Yes

Yes

Execution

Invoked explicitly

Invoked explicitly

Invoked automatically

Use Case

Data computation

Complex operations

Event-driven actions

By using functions, procedures, and triggers effectively, you can encapsulate business logic, enforce rules, and automate tasks within your database. These tools form the backbone of modern DBMS applications, enabling developers and administrators to create powerful and maintainable systems.

---

Mapping Cardinalities in DBMS

Mapping cardinalities, also known as cardinality ratios, define the number of entities from one entity set that can be associated with entities in another entity set through a relationship set. These cardinalities are particularly significant in describing binary relationship sets and are also useful for multi-entity relationships.

For a binary relationship set R between entity sets A and B, the possible mapping cardinalities are as follows:

---

1. One-to-One (1:1)

• Definition: An entity in A is associated with at most one entity in B, and vice versa.
• Example:
  • In a database where employees are assigned to parking spots:
  • Each employee has at most one assigned parking spot.
  • Each parking spot is assigned to at most one employee.
• Diagram Representation:
  • Each entity in A maps to a single entity in B, and each entity in B maps to a single entity in A.

---

2. One-to-Many (1:N)

• Definition: An entity in A can be associated with zero or more entities in B, but an entity in B is associated with at most one entity in A.
• Example:
  • In a database of authors and books:
  • An author can write multiple books.
  • Each book is written by only one author.
• Diagram Representation:
  • Entities in A map to multiple entities in B, but entities in B map to a single entity in A.

---

3. Many-to-One (M:1)

• Definition: An entity in A is associated with at most one entity in B, but an entity in B can be associated with zero or more entities in A.
• Example:
  • In a database of students and courses:
  • Each student can enroll in only one department.
  • A department can have multiple students enrolled.
• Diagram Representation:
  • Entities in A map to a single entity in B, while entities in B can map to multiple entities in A.

---

4. Many-to-Many (M:N)

• Definition: An entity in A can be associated with zero or more entities in B, and vice versa.
• Example:
  • In a database of students and courses:
  • A student can enroll in multiple courses.
  • A course can have multiple students enrolled.
• Diagram Representation:
  • Multiple entities in A map to multiple entities in B, and vice versa.

---

Visual Representation of Mapping Cardinalities

One-to-One (1:1):

CREATE FUNCTION function_name (parameter_list)
RETURNS return_type
AS
BEGIN
    -- Function logic
    RETURN value;
END;

One-to-Many (1:N):

CREATE FUNCTION calculate_total_price(quantity INT, price_per_unit DECIMAL)
RETURNS DECIMAL
AS
BEGIN
    RETURN quantity * price_per_unit;
END;

Many-to-One (M:1):

CREATE PROCEDURE procedure_name (parameter_list)
AS
BEGIN
    -- Procedure logic
END;

Many-to-Many (M:N):

CREATE PROCEDURE update_salary(employee_id INT, new_salary DECIMAL)
AS
BEGIN
    UPDATE employees
    SET salary = new_salary
    WHERE id = employee_id;
END;

---

Importance of Mapping Cardinalities

1. Database Design: Mapping cardinalities help in designing efficient relational schemas by defining clear relationships between entity sets.
2. Data Integrity: Ensure that the relationships conform to real-world constraints.
3. Query Optimization: Knowing the cardinality helps optimize queries for better performance.
4. E-R Models: Play a crucial role in Entity-Relationship diagrams, making relationships explicit.

Mapping cardinalities are foundational to understanding how entities interrelate within a database and provide the structural basis for defining robust and scalable database schemas.

The above is the detailed content of Introduction to Database Management Systems. For more information, please follow other related articles on the PHP Chinese website!