mysql> use test
Database changed
mysql> create table conc ( num1 int, num2 int );
Query OK, 0 rows affected (0.02 sec)

mysql> insert into conc values (1,2),(3,4),(1,2),(3,4);
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> select distinct(val) from ( select concat(num1, num2) val from conc ) c;
+------+
| val  |
+------+
| 12   |
| 34   |
+------+
2 rows in set (0.00 sec)