update
[......]
where
[....]
and   (groups.group_id in  (select groups.group_id
from groups  start with group_id = P_BEGINGROUP
connect by prior group_id = parentgroup_id ))

UPDATE MYTABLE T1 SET
 T1.SOMEFILED = '<VALUE>'
WHERE (T1.GROUP_ID IN (WITH RECURSIVE STMT AS (
 SELECT TB1.ID, TB2.PARENT_ID FROM TREE_TABLE TB1 WHERE (TB1.PARENT_ID = '<VALUE>')
 UNION ALL
 SELECT TB2.ID, TB2.PARENT_ID FROM TREE_TABLE TB2, STMT TB3 WHERE (TB2.PARENT_ID = TB2.ID)
)SELECT TREE.ID FROM STMT TREE)
 )

EXECUTE BLOCK (
 Q_NEW_VALUE VARCHAR(10)
,Q_PARENT_GROUP_ID INT
)AS
 DECLARE VARIABLE P_GROUP_IDS BLOB SUB_TYPE 1 SEGMENT SIZE 80;
BEGIN
 -- Получаем список ID групп с учетом вложенности.
 WITH RECURSIVE STMT AS (
   SELECT TB1.ID, TB2.PARENT_ID FROM TREE_TABLE TB1 WHERE (TB1.PARENT_ID = :Q_PARENT_GROUP_ID)
   UNION ALL
   SELECT TB2.ID, TB2.PARENT_ID FROM TREE_TABLE TB2, STMT TB3 WHERE (TB2.PARENT_ID = TB2.ID)
 )SELECT '~' || LIST(TREE.ID, '~') || '~' FROM STMT TREE INTO :P_GROUP_IDS;

 -- Обновляем набор данных
 UPDATE MYTABLE T1 SET
   T1.SOMEFILED = :Q_NEW_VALUE
 WHERE (:P_GROUP_IDS containing '~'||T1.GROUP_ID||'~');
END