Код:
mysql> create table t ( a int, b int);
Query OK, 0 rows affected (0.01 sec)
mysql> insert into t (a,b) values (10,20),(40,10),(50,60),(40,20);
Query OK, 4 rows affected (0.01 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> select * from t;
+------+------+
| a | b |
+------+------+
| 10 | 20 |
| 40 | 10 |
| 50 | 60 |
| 40 | 20 |
+------+------+
4 rows in set (0.00 sec)
mysql> select if(a>b,b,a) as r from t;
+------+
| r |
+------+
| 10 |
| 10 |
| 50 |
| 20 |
+------+
4 rows in set (0.00 sec)
Диалект SQL ты не уточнял. Решение дано для mysql.