WITH tb AS
(SELECT 1 a,'aa' b FROM dual
UNION ALL
SELECT 1,'bb' FROM dual
)
SELECT a,max(substr(sys_connect_by_path(b,'+'),2))
FROM 
(
       SELECT a,b,row_number()over(PARTITION BY a ORDER BY a) rn
       FROM tb
) START WITH rn=1
  CONNECT BY rn-1= PRIOR rn AND a=PRIOR a
  GROUP BY a

--result:

1    aa+bb

------解决方案--------------------
SQL code
with tb as(  
     select 'A' grade,'XX'name from dual union all  
     select 'A','XY' from dual union all  
     select 'A','YY' from dual union all  
     select 'B','aa' from dual union all  
     select 'B','bb' from dual)  
select grade,substr(max(sys_connect_by_path(name,';')),2) name  
from (select grade,name,row_number() over(partition by grade order by name) rn from tb)  
start with rn=1  
connect by  rn= prior rn+1  and   
            connect_by_root(grade)=grade  
group by grade;  
--   
GRADE NAME  
----- --------------------------------------------  
A     XX;XY;YY  
B     aa;bb  
--  
10g的实现：  
with t as(  
     select 'A' grade,'XX'name from dual union all  
     select 'A','XY' from dual union all  
     select 'A','YY' from dual union all  
     select 'B','aa' from dual union all  
     select 'B','bb' from dual)  
select grade,wmsys.wm_concat(name) name  
from t  
group by grade;  
--  
GRADE NAME  
----- --------------------------------------------  
A     XX,XY,YY  
B     aa,bb

------解决方案--------------------

我的示例代码，亲测，呵呵。
SQL code

-- 假定NO列为主键
CREATE TABLE A
(
    NO      VARCHAR(2),
    Text    VARCHAR(20)
);

CREATE TABLE B
(
    NO      VARCHAR(2),
    Text    VARCHAR(20)
);

INSERT INTO A VALUES('1', 'aa');
INSERT INTO A VALUES('2', 'xx');

INSERT INTO B VALUES('1', 'bb');
INSERT INTO B VALUES('2', 'yy');


SELECT NO AS NO, a.Text || '+' ||b.Text AS Text 
FROM A JOIN B USING(NO);