Introduction to SQL

SQL (Structured Query Language) is used to manage and query data in databases. MySQL is one of the most popular database systems in the world.

What Can SQL Do?

• Create, read, update, and delete data (CRUD)
• Create and modify database tables
• Join data from multiple tables
• Filter and sort data
• Aggregate data (count, sum, average)

Syntax

MYSQL

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-- Select all columns from a table
SELECT * FROM students;

-- Select specific columns
SELECT name, email FROM students;

-- Filter with WHERE
SELECT * FROM students WHERE age > 18;

-- Insert data
INSERT INTO students (name, email) VALUES ("Ali", "ali@email.com");

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

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

Create Your First Table

SQL

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CREATE DATABASE IF NOT EXISTS shop;
USE shop;

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(100) NOT NULL,
    email VARCHAR(150) UNIQUE NOT NULL,
    age INT,
    city VARCHAR(50) DEFAULT "Unknown",
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

DESCRIBE users;

Try It Yourself

Practice

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Exercise

Write a SQL query to select all columns from a table called "courses".

Answer

SELECT * FROM courses;

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Exercise

Write a query to insert a student named "Ali" with age 25.

Answer

INSERT INTO students (name, age) VALUES ("Ali", 25);

Quick Quiz

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What does CRUD stand for?

Create, Read, Update, Delete

Computer, Read, Upload, Download

Code, Run, Use, Debug

Create, Run, Update, Drop

CRUD stands for Create, Read, Update, Delete — the four basic database operations.

Retry Quiz

Interview Questions

What is the difference between WHERE and HAVING?

WHERE filters rows before grouping/aggregation. HAVING filters groups after GROUP BY. You cannot use aggregate functions (COUNT, SUM, etc.) in WHERE but you can in HAVING.

Next Lesson

SELECT & WHERE