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 14

LEFT JOINs

Keeping all left table rows — even when there is no match.

IN THIS CHAPTER

LEFT JOIN returns ALL rows from the left table
Matching rows from the right table are included
If no match is found, NULL values are returned
Useful to find missing relationships (e.g., customers with no orders)
LEFT JOIN + IS NULL helps detect unmatched records

🌟 Think of it this way: You have a list of all customers and a list of orders. A LEFT JOIN ensures every customer appears — even if they never placed an order. Missing orders will show as NULL.

14.1 - LEFT JOIN

Returns ALL rows from the left table (customers). Unmatched rows show NULL on the right side.

SQL

SELECT c.name, o.order_id, o.total
FROM customers c
LEFT JOIN orders o ON c.customer_id = o.customer_id
ORDER BY c.name;

14.2 - Find Customers Who Never Ordered

Classic LEFT JOIN + IS NULL patter

SELECT c.name
FROM customers c
LEFT JOIN orders o ON c.customer_id = o.customer_id
WHERE o.order_id IS NULL;

✅ Pro Tips: This pattern — LEFT JOIN + WHERE right_side IS NULL — is one of the most frequently used patterns in analytics. 'Users who signed up but never purchased', 'products never sold', 'employees not assigned to any project'.

✅ Pro Tip: LEFT JOIN + IS NULL is widely used to find missing data — like users who never logged in, customers with no orders, or products that were never sold.

Included Tables in Exercise

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

Exercise 👇

Exercise:

Tasks

1.👉Get all customers with their order IDs (include customers with no orders)

2.Show order totals with customer names (include NULLs for no orders)

3.Orders above 5000 (note: customers without orders will be excluded due to WHERE)

4.Only completed orders (note: behaves like INNER JOIN due to filtering)

5.Sort customers by order amount

6.Count orders per customer (including 0 orders)

7.Total spending per customer (NULL becomes 0 in understanding)

8.Customers with more than 1 order

9.Latest order date per customer

10.Average order value per customer

11.Find customers who have NOT placed any orders

12.Join 3 tables using LEFT JOIN

13.Product names with customer orders (including NULLs)

14.Total quantity ordered per product (including unsold products)

15.Customers who bought Electronics (LEFT JOIN + filter)

Stuck? Read this task's