SQL> SELECT 1, (SELECT '100'
  2  FROM DUAL
  3  UNION
  4  SELECT '101'
  5  FROM DUAL
  6  UNION
  7  SELECT '103'
  8  FROM DUAL)
  9  FROM DUAL
 10  UNION ALL
 11  SELECT 2, (SELECT '200'
 12  FROM DUAL
 13  UNION
 14  SELECT '201'
 15  FROM DUAL
 16  UNION
 17  SELECT '203'
 18  FROM DUAL)
 19  FROM DUAL;
SELECT 1, (SELECT '100'
           *
ERROR at line 1:
ORA-01427: single-row subquery returns more than one row

SQL> SELECT 1, b
  2  from (SELECT '100' b
  3  FROM DUAL
  4  UNION
  5  SELECT '101'
  6  FROM DUAL
  7  UNION
  8  SELECT '103'
  9  FROM DUAL),
 10  DUAL
 11  UNION ALL
 12  SELECT 2, b
 13  from (SELECT '200' b
 14  FROM DUAL
 15  UNION
 16  SELECT '201'
 17  FROM DUAL
 18  UNION
 19  SELECT '203'
 20  FROM DUAL ),
 21  DUAL
 22  /
         1 B
---------- ---
         1 100
         1 101
         1 103
         2 200
         2 201
         2 203

6 rows selected.