mysql> create table t ( id int(10) not null default 0, po int(10) not null default 0);
Query OK, 0 rows affected (0.04 sec)

mysql> insert into t set id=1,po=1;
Query OK, 1 row affected (0.00 sec)

mysql> insert into t set id=2,po=1;
Query OK, 1 row affected (0.00 sec)

mysql> SELECT po,id FROM t GROUP BY po;
+----+----+
| po | id |
+----+----+
|  1 |  1 |
+----+----+
1 row in set (0.01 sec)