SQL
Identify Records Lacking Related Data with LEFT JOIN
Discover records in one table that do not have corresponding entries in a related table by combining a LEFT JOIN with a WHERE clause checking for NULLs.
SELECT
c.customer_id,
c.customer_name,
c.email
FROM
customers c
LEFT JOIN
orders o ON c.customer_id = o.customer_id
WHERE
o.order_id IS NULL;
How it works: This query efficiently finds all customers who have not placed any orders. It uses a `LEFT JOIN` to combine every customer from the `customers` table with any matching orders from the `orders` table. If a customer has no corresponding order, the columns from the `orders` table (like `o.order_id`) will be `NULL`. The `WHERE o.order_id IS NULL` condition then filters the results to only show those customers without any orders, making it ideal for identifying inactive users or data inconsistencies.