Relational Databases and SQL

Relational Databases and SQL

Introduction

Relational databases are a type of database that stores data in tables with rows and columns. Each row represents a record, and each column represents a field or attribute. These tables are related to each other through shared values, forming a structured and organized system for storing and retrieving information.

SQL (Structured Query Language) is the standard language used to interact with relational databases. It provides a set of commands for defining, querying, and manipulating data within these databases.

Key Concepts

• Tables: Organize data into rows and columns.
• Rows: Represent individual records.
• Columns: Represent attributes or fields.
• Primary Key: Uniquely identifies each record in a table.
• Foreign Key: Links tables together by referencing a primary key in another table.
• Relationships: Define how tables are connected and related to each other.

SQL Commands

Data Definition Language (DDL): Used to create, modify, and delete database objects.

• CREATE TABLE: Creates a new table. CREATE TABLE Customers ( CustomerID INT PRIMARY KEY, FirstName VARCHAR(255), LastName VARCHAR(255), Email VARCHAR(255) );
• ALTER TABLE: Modifies an existing table. ALTER TABLE Customers ADD COLUMN PhoneNumber VARCHAR(20);
• DROP TABLE: Deletes a table. DROP TABLE Customers;

Data Manipulation Language (DML): Used to insert, update, and delete data in tables.

• INSERT INTO: Inserts new data into a table. INSERT INTO Customers (CustomerID, FirstName, LastName, Email) VALUES (1, 'John', 'Doe', '[email protected]');
• UPDATE: Modifies existing data in a table. UPDATE Customers SET Email = '[email protected]' WHERE CustomerID = 1;
• DELETE FROM: Deletes data from a table. DELETE FROM Customers WHERE CustomerID = 1;

Data Query Language (DQL): Used to retrieve data from tables.

• SELECT: Retrieves data from a table. SELECT * FROM Customers;
• WHERE: Filters data based on a condition. SELECT * FROM Customers WHERE FirstName = 'John';
• ORDER BY: Sorts data based on a column. SELECT * FROM Customers ORDER BY LastName;
• JOIN: Combines data from multiple tables. SELECT Customers.FirstName, Orders.OrderID FROM Customers JOIN Orders ON Customers.CustomerID = Orders.CustomerID;

Advantages of Relational Databases

• Data Integrity: Ensures data consistency and accuracy through constraints and relationships.
• Data Security: Provides mechanisms for access control and data protection.
• Data Standardization: Uses a standardized language (SQL) for managing data.
• Data Scalability: Can handle large amounts of data efficiently.

Conclusion

Relational databases and SQL are essential tools for storing, managing, and accessing data in a structured and organized manner. Understanding these concepts is crucial for working with databases and developing efficient data-driven applications.