これは機能します
select * from (
(select 1 a,2 b,3 c) t1 left join (select null a,2 b,5 c) t2 on (t1.b=t2.b)
);
または、
select * from (
(select 1 a,2 b,3 c) t1 left join (select null a,2 b,5 c) t2 using (b)
);
どちらも結果として
+---+---+---+------+---+---+
| a | b | c | a | b | c |
+---+---+---+------+---+---+
| 1 | 2 | 3 | NULL | 2 | 5 |
+---+---+---+------+---+---+
1 row in set (0.00 sec)