Я считаю, что это самое простое правильное решение:
SELECT MAX (phone_number) as phone_number, id
FROM phone_numbers
GROUP BY id
HAVING COUNT (*) = 1
С данными испытаний:
WITH phone_numbers AS
(
SELECT '09123456789' AS phone_number , 1234 AS ID FROM dual
UNION ALL
SELECT '09876543210' AS phone_number , 1234 AS ID FROM dual
UNION ALL
SELECT '09012463579' AS phone_number , 5678 AS ID FROM dual
)
SELECT MAX (phone_number) as phone_number, id
FROM phone_numbers
GROUP BY id
HAVING COUNT (*) = 1
phone_number | id |
----------------------
09012463579 | 5678 |
Надеюсь, я помог!