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 2

Your Sample Database

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

IN THIS CHAPTER

The five tables we will query throughout every chapter
How tables relate to each other via primary and foreign keys
How to read and interpret a database schema

Table: employees

emp_id	name	department	salary	manager_id	hire_date
1	Arjun	IT	85000		2020-06-15
2	Priya	IT	72000	1	2021-03-10
3	Rahul	Sales	55000	5	2022-01-20
4	Sneha	HR	60000	5	2021-07-08
5	Vikram	Sales	90000		2019-11-01
6	Divya	IT	68000	1	2023-02-14
7	Kiran	HR	58000	4	2022-08-30
8	Meera	Sales	62000	5	2023-05-12

Table: products

product_id	name	category	price	stock
1	iPhone 15	Electronics	79999	50
2	SQL Book	Books	499	200
3	Headphones	Electronics	2999	75
4	Notebook	Stationery	99	500
5	Laptop	Electronics	65000	20
6	Pen Set	Stationery	149	0
7	Mouse	Electronics	599	25

Table: customers

customer_id	name	city	country
1	Ananya	Hyderabad	India
2	Rohan	Bangalore	India
3	Sam	Mumbai	India
4	Lisa	London	UK
5	Ravi	Delhi	India

Table: orders

order_id	customer_id	order_date	total	status
1	1	2024-01-15	79999	completed
2	2	2024-01-20	3498	completed
3	1	2024-02-10	65000	completed
4	3	2024-02-14	499	pending
5	2	2024-03-01	248	completed
6	5	2024-03-15	2999	shipped
7	1	2024-04-05	149	completed

Table: orders_items

item_id	order_id	product_id	qty	unit_price
1	1	1	1	79999
2	2	3	1	2999
3	2	2	1	499
4	3	5	1	65000
5	4	2	1	499
6	5	4	2	99
7	5	6	1	50
8	6	3	1	2999

How the Tables Connect

Table Name	Description	Key Columns
orders -> customers	order.customer_id	customer.customer_id
order_items -> orders	order_items.order_id	order.order_id
order-items -> products	order_items.product_id	product.product_id
employees -> employees	employees.manager_id	employee.emp_id (self-reference)

How the Tables Connect

Table Name	Description	Key Columns
orders -> customers	order.customer_id	customer.customer_id
order_items -> orders	order_items.order_id	order.order_id
order-items -> products	order_items.product_id	product.product_id
employees -> employees	employees.manager_id	employee.emp_id (self-reference)

💡 Note: Lisa (customer_id 4) has no orders. Pen Set (product_id 6) has stock = 0. These "edge cases" are intentional — they are used to illustrate LEFT JOIN and NULL behaviour in later chapters.