[Solved] Which of the following SQL queries correctly finds customers without orders using LEFT JOIN? — Ans: SELECT c.id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id WHERE... | SQL

SQL - LEFT and RIGHT JOIN

Which of the following SQL queries correctly finds customers without orders using LEFT JOIN?

ASELECT c.id FROM customers c RIGHT JOIN orders o ON c.id = o.customer_id WHERE c.id IS NULL;

BSELECT c.id FROM customers c INNER JOIN orders o ON c.id = o.customer_id WHERE o.customer_id IS NULL;

CSELECT c.id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id WHERE c.id IS NULL;

DSELECT c.id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id WHERE o.customer_id IS NULL;

Step-by-Step Solution

Solution:

Step 1: Identify correct JOIN type
LEFT JOIN keeps all customers and matches orders; unmatched orders will be NULL.
Step 2: Filter unmatched orders
WHERE o.customer_id IS NULL selects customers without orders.
Final Answer:
SELECT c.id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id WHERE o.customer_id IS NULL; -> Option D
Quick Check:
LEFT JOIN + right table NULL = unmatched left rows [OK]

Quick Trick: Use LEFT JOIN and check right table key IS NULL [OK]

Common Mistakes:

MISTAKES

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Which of the following SQL queries correctly finds customers without orders using LEFT JOIN?