Database & SQL

Last updated: 2025-09-25 02:02:49

SQL Database Fundamentals

SQL (Structured Query Language) is used to manage relational databases.

Basic Queries

-- Select data
SELECT * FROM users;
SELECT name, email FROM users WHERE age > 18;

-- Insert data
INSERT INTO users (name, email, age) 
VALUES ("John Doe", "john@email.com", 25);

-- Update data
UPDATE users SET age = 26 WHERE id = 1;

-- Delete data
DELETE FROM users WHERE id = 1;

Table Operations

-- Create table
CREATE TABLE users (
    id INT PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(100) NOT NULL,
    email VARCHAR(100) UNIQUE,
    age INT,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

-- Modify table
ALTER TABLE users ADD COLUMN phone VARCHAR(20);
ALTER TABLE users DROP COLUMN phone;

Joins

-- Inner Join
SELECT u.name, p.title 
FROM users u 
INNER JOIN posts p ON u.id = p.user_id;

-- Left Join
SELECT u.name, p.title 
FROM users u 
LEFT JOIN posts p ON u.id = p.user_id;