SQL
Find Records Without Any Related Entries (Anti-Join with NOT EXISTS)
Identify parent records that do not have any corresponding child records in a related table using the efficient NOT EXISTS subquery pattern, useful for data cleanup.
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
);
How it works: This SQL query acts as an 'anti-join' to find records in the `customers` table that do not have any corresponding entries in the `orders` table. The `NOT EXISTS` clause evaluates to true if the subquery returns no rows. This is highly efficient for identifying 'orphaned' records or customers who haven't placed any orders, often performing better than `LEFT JOIN ... WHERE related_id IS NULL` for large datasets.