SELECT c1.customer_id, c1.first_name, c1.last_name, c2.customer_id, c2.first_name, c2.last_name
FROM customer c1
LEFT JOIN customer c2 ON c1.customer_id = c2.customer_id
UNION
SELECT c1.customer_id, c1.first_name, c1.last_name, c2.customer_id, c2.first_name, c2.last_name
FROM customer c1
RIGHT JOIN customer c2 ON c1.customer_id = c2.customer_id
WHERE c1.customer_id IS NULL
AND c1.first_name = c2.last_name
ORDER BY c.1customer_id