SQL: Find Customers Who Have Gone Quiet with a LEFT JOIN

Why this matters

Customers rarely announce that they have left; they just stop ordering. At Lakeside Office Supply (made up) nobody noticed a dental practice had gone quiet until the year-end review, months too late for a call to help.

The owner now wants a list every Monday: every customer with no order since 1 September, with the account manager next to each name. In this module you build that list with two tables and a LEFT JOIN, and see why a plain JOIN would have hidden the one customer who never ordered at all.

What you’ll build

A customers table (five customers, two account managers), the orders table, and a call-list query.

With the sample data: Elm Street Law (never ordered) and Harbor Accounting (last order 19 August) for Ana, and Maple Dental (27 August) for Sam.

What you need

Nothing to start the practiceThe practice is built from real DB Fiddle screens and runs in this page.
A browser, to do it for realDB Fiddle is free and needs no account.

Concepts

3 ideas you’ll use in the practice.

Two tables, one link

Real systems keep customers and orders in separate tables. A JOIN puts them side by side: FROM customers c JOIN orders o ON o.customer = c.name matches every order to its customer. The short names c and o (aliases) save typing.

JOIN drops, LEFT JOIN keeps

A plain JOIN keeps only rows that match on both sides, so a customer with no orders disappears. A LEFT JOIN keeps every row from the left table (customers) and fills the order columns with null where there is no match.

Any "who has not ..." question (not ordered, not paid, not replied) needs a LEFT JOIN.

MAX and HAVING for "last activity"

MAX(o.order_date) gives each customer's latest order date. HAVING MAX(o.order_date) < '2026-09-01' OR MAX(o.order_date) IS NULL keeps customers whose latest order is older than the cut-off, or who never ordered.

Practice

Do every step yourself in a practice copy of the real tool. It checks your work, and “Do it for me” finishes any step you get stuck on.

DB FiddleSQL: Find Customers Who Have Gone Quiet with a LEFT JOIN17 steps · ~15 min · nothing touches your real accounts

Start the practice →Prefer to read? Step-by-step guide

Do it for real

Now repeat it in your own account. Tick each step as you go.

Open db-fiddle.com (MySQL, no account). In Schema SQL type the two CREATE TABLE customers lines and the five customers, then the orders table and its eight orders.
In Query SQL type SELECT * FROM customers; and click Run to check it.
Replace the query: type SELECT c.name, c.manager, MAX(o.order_date) AS last_order, then FROM customers c, then JOIN orders o ON o.customer = c.name, then GROUP BY c.name, c.manager; and click Run.
Notice the customer who never ordered is missing. Type LEFT and a space in front of JOIN and click Run again.
Delete the last semicolon, press Enter and type HAVING MAX(o.order_date) < '2026-09-01', then OR MAX(o.order_date) IS NULL, then ORDER BY last_order; and click Run.
At work, set the date to 90 days ago and send the list to each account manager every Monday.

Quick check

3 questions to check you’ve got it.

1. Why was Elm Street Law missing from the first JOIN result?

It is spelled wrong
It has no orders, and a plain JOIN keeps only matching rows
Its manager is Ana

Show the answer

It has no orders, and a plain JOIN keeps only matching rows. A JOIN keeps only customers with at least one order. A LEFT JOIN keeps every customer.

2. What does MAX(o.order_date) give you per customer?

Their biggest order
Their latest order date
Their number of orders

Show the answer

Their latest order date. MAX on a date column is the most recent date.

3. With the sample data, which customers are on the call list?

Cobalt Legal and Northside Realty
Elm Street Law, Harbor Accounting and Maple Dental
Only Elm Street Law

Show the answer

Elm Street Law, Harbor Accounting and Maple Dental. Cobalt Legal (22 September) and Northside Realty (9 September) ordered after 1 September; the other three did not.

Course complete

You reached the end of SQL for Business Reports Jump Start

That completes the SQL jump start: a monthly report, your top customers and a call list, all straight from the data.

Pick your next course →