SQL Programming Resources on Telegram: “SQL Interview Series

SQL Interview Series — Part 8

📌 Question 8: Find Customers Who Have Never Placed an Order

Suppose you have two tables:

Customers

customer_id | customer_name
------------|--------------
1           | Amit
2           | Rahul
3           | Priya
4           | Neha
5           | Raj

Orders

order_id | customer_id
---------|------------
101      | 1
102      | 3
103      | 1
104      | 5

❓ Find all customers who have never placed an order.

💡 Approach

We need to find customers from the "Customers" table who do not have a matching record in the "Orders" table.

A "LEFT JOIN" is a common and interview-friendly approach.

📌 SQL Solution

SELECT
    c.customer_id,
    c.customer_name
FROM Customers c
LEFT JOIN Orders o
    ON c.customer_id = o.customer_id
WHERE o.customer_id IS NULL;

🔎 How does it work?

"LEFT JOIN" keeps every customer, even if they don't have an order.

For customers without an order, the columns from "Orders" become "NULL".

Then:

WHERE o.customer_id IS NULL

keeps only those customers.

📌 Expected Result:

customer_id | customer_name
------------|--------------
2           | Rahul
4           | Neha

⚠️ Interview Trap

You can also solve this using "NOT EXISTS":

SELECT
    c.customer_id,
    c.customer_name
FROM Customers c
WHERE NOT EXISTS (
    SELECT 1
    FROM Orders o
    WHERE o.customer_id = c.customer_id
);

"NOT EXISTS" is often preferred when you specifically want to test whether a matching row exists.

Be careful with "NOT IN" when NULL values can exist in the subquery, because NULL can lead to unexpected results.

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