SQL
Finding Unmatched Records Using LEFT JOIN and IS NULL
Identify records in one table that do not have corresponding entries in another table, a critical technique for data integrity checks and precise reporting without `NOT EXISTS`.
SELECT
p.product_id,
p.product_name
FROM
Products p
LEFT JOIN
Order_Items oi ON p.product_id = oi.product_id
WHERE
oi.product_id IS NULL; -- Filters for products with no matching order items
How it works: This SQL query performs an anti-join to find records in the `Products` table that do not have any corresponding entries in the `Order_Items` table. A `LEFT JOIN` is used to include all products from the left table (`Products`). If a product has no matching `order_item`, the columns from `Order_Items` will be `NULL`. The `WHERE oi.product_id IS NULL` condition then filters for precisely these products, indicating they haven't been ordered.