SQL> drop table t purge;

Table dropped.

SQL> create table t ( x int, y int, z number );

Table created.

SQL>
SQL>
SQL> insert into t values (1,2,0.852 );

1 row created.

SQL> insert into t values (1,3,0.433 );

1 row created.

SQL> insert into t values (1,4,0.538 );

1 row created.

SQL> insert into t values (2,3,0.991 );

1 row created.

SQL> insert into t values (2,4,0.604 );

1 row created.

SQL> insert into t values (3,4,0.597 );

1 row created.

SQL>
SQL> select val, min(z)
  2  from
  3    ( select x val, z from t
  4      union all
  5      select y, z from t
  6    )
  7  group by val;

       VAL     MIN(Z)
---------- ----------
         1       .433
         2       .604
         4       .538
         3       .433

4 rows selected.

SQL>
SQL>

SQL> select id, min(z)
  2  from   t
  3  unpivot (id for tag in (x as 'x', y as 'y'))
  4  group by id;

        ID     MIN(Z)
---------- ----------
         1       .433
         2       .604
         4       .538
         3       .433

4 rows selected.