Find Records Lacking Related Entries with SQL 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
);

-- Or, alternatively, find products that have never been ordered:
SELECT
  p.product_id,
  p.product_name
FROM
  products p
WHERE NOT EXISTS (
  SELECT 1
  FROM order_items oi
  WHERE oi.product_id = p.product_id
);

How it works: This SQL snippet demonstrates how to find customers who have never placed an order using the `NOT EXISTS` clause with a subquery. The outer query selects customers, and the `WHERE NOT EXISTS` condition checks if there is *any* record in the `orders` table for that specific customer. If no such record exists, the customer is included in the result. This pattern is efficient for identifying missing relationships without a full outer join.