SQL subqueries: use one answer inside another

WHERE price > (SELECT AVG(price) FROM products) compares each row with a single computed value.

IN (SELECT customer_id FROM orders) checks membership in a list that another query produces.

Worked example

Which products cost more than the average product?

SELECT name, price
FROM products
WHERE price > (SELECT AVG(price) FROM products)
ORDER BY price DESC;

Common mistakes

• Using = with a subquery that returns several rows.
• Using NOT IN with a list that contains NULL, which returns nothing.