Given an orders table, an order_items table, and a products table, write a query to find products that were never sold.

Assuming the tables are defined as:

• products(product_id, name, ...)
• orders(order_id, ...)
• order_items(order_item_id, order_id, product_id, quantity, ...)

You can find products that never appear in any order_item record using a LEFT JOIN or NOT EXISTS. Example with LEFT JOIN:

SELECT p.product_id, p.name

FROM products p

LEFT JOIN order_items oi ON p.product_id = oi.product_id

WHERE oi.order_item_id IS NULL;

Alternatively, using NOT EXISTS:

SELECT p.product_id, p.name

FROM products p

WHERE NOT EXISTS (

SELECT 1 FROM order_items oi WHERE oi.product_id = p.product_id

);

Both queries return the same result. Complexity is O(P + OI) where P is the number of products and OI is the number of order_items, assuming appropriate indexes on product_id in order_items. The LEFT JOIN version may be slightly faster if the database can use an index on order_items.product_id to quickly find null matches.