+------------+----------+
|    id      |  text    |
+------------+----------+
| 1          | message1 |
| 2          | message2 |
| 3          | message3 |
+------------+----------+

+------------+-------------+
|    id      |  message_id |
+------------+-------------+
| 1          | 1           |
| 2          | 2           |
+------------+-------------+

SELECT
    *
FROM
    message AS a
        JOIN
    user AS b ON a.id = b.message_id AND b.id = 1

UNION ALL

SELECT
    *
FROM
    message AS a
        LEFT JOIN
    user AS b ON a.id = b.message_id
WHERE
    b.id IS NULL

+------------+-------------+
|    id      |  message_id |
+------------+-------------+
| 1          | 1           |
| 2          | 2           |
| 1          | 3           |
| 2          | 3           |
+------------+-------------+