CREATE TABLE TW_GROUP (Article NUMBER, Subgrp NUMBER)

INSERT INTO TW_GROUP VALUES (1, 20)

INSERT INTO TW_GROUP VALUES (2, 23)

CREATE TABLE TW_GROUP_TREE (Grp NUMBER, Parent NUMBER, Code NUMBER)

INSERT INTO TW_GROUP_TREE VALUES (20, 0, 6)

INSERT INTO TW_GROUP_TREE VALUES (23, 0, NULL)

INSERT INTO TW_GROUP_TREE VALUES (0, NULL, 99)

SELECT Article, 
       (SELECT FIRST_VALUE(CODE) OVER(ORDER BY Level) 
          FROM TW_GROUP_TREE 
         WHERE Code IS NOT NULL 
           AND RowNum = 1 
    CONNECT BY PRIOR Parent = Grp 
    START WITH Grp  = (SELECT SubGrp 
                         FROM TW_GROUP 
                        WHERE Article  = u1.Article)) AS Code_with_Subquery, 
       (SELECT FIRST_VALUE(CODE) OVER(ORDER BY Level) 
          FROM TW_GROUP_TREE 
         WHERE Code IS NOT NULL 
           AND RowNum = 1 
    CONNECT BY PRIOR Parent = Grp 
    START WITH Grp  = u1.SubGrp) AS Code_without_Subquery 
  FROM (SELECT 1 Article, 
               (SELECT SubGrp 
                 FROM TW_GROUP 
                WHERE Article  = 1) AS SubGrp 
          FROM DUAL 
        UNION ALL 
        SELECT 2 Article, 
               (SELECT SubGrp 
                 FROM TW_GROUP 
                WHERE Article  = 2) AS SubGrp 
          FROM DUAL) u1