EXISTS.

EXISTS (SELECT 1 FROM ... WHERE ...) is true when the subquery returns at least one row — the row contents do not matter, hence SELECT 1. The subquery is tied to the outer row by a condition such as o.customer_id = c.id, so the check runs per customer.

It answers "does this customer have at least one …?". EXISTS yields only true/false and does not multiply rows. A NULL inside the subquery does not confuse it.

Common mistakes: forgetting the linking condition (the subquery then does not depend on the customer and EXISTS is the same for everyone); using COUNT(*) > 0 instead of EXISTS; mixing up the aliases c and o.

Small example:

SELECT c.id, c.name FROM customers c WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id AND o.total >= 50);