SELECT IFNULL(customer_id,customer_id2), name, phone_num FROM(
	   SELECT c.customer_id, name, p.customer_id AS customer_id2, phone_num
	   FROM customers c
	   LEFT JOIN phone_numbers p
	   ON c.customer_id = p.customer_id
	   UNION (
	   SELECT *
	   FROM customers c
	   RIGHT JOIN phone_numbers p
	   ON c.customer_id = p.customer_id)
) a