주문(orders)을 한 번도 한 적 없는 고객(customer)만 조회하려 할 때 가장 올바른 쿼리는?

1. 1SELECT c.* FROM customer c INNER JOIN orders o ON c.cid = o.cid WHERE o.oid IS NULL
2. 2SELECT c.* FROM customer c LEFT OUTER JOIN orders o ON c.cid = o.cid WHERE o.oid IS NULL
3. 3SELECT c.* FROM customer c RIGHT OUTER JOIN orders o ON c.cid = o.cid WHERE o.oid IS NULL
4. 4SELECT c.* FROM customer c, orders o WHERE c.cid = o.cid AND o.oid IS NULL