Ask whether a matching row exists

EXISTS (subquery) is true when the subquery returns at least one row. It is false when it returns none. A correlated subquery can refer to the current outer row.

SELECT c.id
FROM customers AS c
WHERE EXISTS (
  SELECT 1
  FROM orders AS o
  WHERE o.customer_id = c.id
);

The result contains at most one row for each customer, even if that customer has many orders. The subquery's selected values are usually irrelevant; the question is whether a row exists.