DataBolt

All Lessons

Introduction to SQL

SQL Lesson 1: Your Sample Database

SQL Lesson 2: SELECT — Reading data

SQL Lesson 3: WHERE — Filtering rows

SQL Lesson 4: AND, OR, NOT

SQL Lesson 5: BETWEEN, IN, LIKE

SQL Lesson 6: NULL — The Mystery Value

SQL Lesson 7: ORDER BY

SQL Lesson 8: LIMIT & OFFSET

SQL Lesson 9: Aggregate Functions

SQL Lesson 10: GROUP BY

SQL Lesson 11: HAVING

SQL Lesson 12: INNER JOINs

SQL Lesson 13: LEFT JOINs

SQL Lesson 14: RIGHT JOINs

SQL Lesson 15: SELF JOINs

SQL Lesson 16: UNION JOINs

SQL Lesson 17: Joining Multiple Tables

SQL Lesson 18: Subqueries

SQL Lesson 19: CTEs (WITH)

SQL Lesson 20: CASE Statements

SQL Lesson 21: Window Functions

SQL Lesson 22: String Functions

SQL Lesson 23: Date & Time Functions

SQL Lesson 24: INSERT, UPDATE, DELETE

SQL Lesson 25: CREATE TABLE & DDL

SQL Lesson 26: Indexes & Performance

SQL Lesson 27: Transactions & ACID

SQL Lesson 28: SQL Execution Order

SQL Lesson 29: 50 Practice Problems

CHAPTER 1

What is SQL?

And why is it one of the most valuable skills you can learn in tech?

IN THIS CHAPTER

What a database is and why every app needs one
The difference between a database, a table, a row, and a column
Why SQL has remained the #1 data language for 50+ years
Key terminology you will use throughout your career

Every App you Love Runs on a Database

When you scroll Instagram, the app sends a SQL query that looks something like:

SELECT post_id, image_url, caption, like_count 
FROM posts 
WHERE user_id IN (
    Select following_id FROM followers WHERE user_id = ?;
)
ORDER BY created_at DESC
LIMIT 20;

This query runs millions of times per second at scale, SQL is the universal language of data.

Key Terminology

Term	What It Means	Real-World Analogy
Database	A container holding all related tables	All of Swiggy's data - users, restaurants, orders
Table	One entity stored as rows + columns	The "orders" spreadsheet inside Swiggy's database
Row / Record	A single record in a table	A single order placed by Ananya
Column / Field	One attribute of each row	The "total_amount" column in the orders table
Primary Key	Unique ID - no two rows share it	Your Aadhaar number
Foreign Key	A column that links to another table's primary key	order.customer_id --> customer.customer_id
Query	A SQL statement asking a question or giving a command	SELECT name FROM employees WHERE salary > 7000
Schema	The structure definition of a database	The blueprint listing all tables and their columns

💡 Engineering Insight: At Google, databases can have tables with trillions of rows annd schemas with hundreds of tables. Yet the SQL you write today is the same SQL engineers write at Google - the language scales from 8 rows to 8 trillion.