Finding Customers Who Have Orders but No Payments Using JOINs
To find customers who have placed orders but have not made any payments, you can use a LEFT JOIN between orders and payments and then filter rows where the payment does not exist (IS NULL). The key idea is: JOIN all orders, then keep only those without matching payments.
The LEFT JOIN keeps all orders even if a payment doesn't exist. Rows where payment_id is NULL indicate the order has no payment.
Use LEFT JOIN + IS NULL when you also want to fetch data from the main tables (e.g., order_id).
Use NOT EXISTS when you only need to check absence—it avoids row multiplication and can be more efficient.
Avoid using RIGHT JOIN or FULL JOIN for this; LEFT JOIN or NOT EXISTS is cleaner and more readable.