𝗜𝗻𝘁𝗲𝗿𝘃𝗶𝗲𝘄𝗲𝗿:
You have 2 minutes to solve this SQL query.

Find customers who have never placed an order.

Tables:

customers(customer_id, customer_name)

orders(order_id, customer_id, order_date)


𝗠𝗲: Challenge accepted! 💪

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;

💡 Explanation:

This query uses a LEFT JOIN to return all customers, whether or not they have placed an order.

• LEFT JOIN keeps every customer in the result.
• Customers without matching records in the orders table will have NULL values.
• The WHERE o.customer_id IS NULL condition filters only customers who have never placed an order.

This question tests your understanding of:
✅ LEFT JOIN
✅ Finding missing records
✅ NULL handling

🎯 Expected Output Example

Customer ID Customer Name

105 Alice
112 David
118 Sarah


🚀 Questions about finding unmatched records are very common in interviews. Practice using:

- LEFT JOIN ... IS NULL

- NOT EXISTS

- NOT IN (carefully, because NULL values can affect results)

Among these, NOT EXISTS is often preferred for correctness and performance in many databases.

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