허접하지만 ..^^

create table a (
idx char(5), title varchar(20), name varchar(20));

create table b (
idx char(5), filename varchar(20), aidx varchar(20));

insert into a values('1', 'a1', 'a1_n');
insert into b values('1', 'a1_fn1', 'a1_fx1');
insert into b values('1', 'a1_fn2', 'a1_fx2');
insert into b values('1', 'a1_fn3', 'a1_fx3');

insert into a values('2', 'a2', 'a2_n');
insert into b values('2', 'a2_fn1', 'a2_fx1');
insert into b values('2', 'a2_fn2', 'a2_fx2');

SELECT IDX,TITLE,NAME,MAX(F1),MAX(F2),MAX(F3)
FROM
(SELECT IDX,TITLE,NAME,
       MAX(CASE WHEN RN=1 THEN FILENAME ELSE '' END) F1,
       MAX(CASE WHEN RN=2 THEN FILENAME ELSE '' END) F2,
       MAX(CASE WHEN RN=3 THEN FILENAME ELSE '' END) F3
FROM
(SELECT A.IDX,TITLE,NAME,FILENAME,RANK() OVER(PARTITION BY A.IDX ORDER BY FILENAME) RN
  FROM A A,B B
 WHERE A.IDX=B.IDX
) A
GROUP BY IDX,TITLE,NAME,FILENAME
) A
GROUP BY IDX,TITLE,NAME
;

이렇게 하는건 어떨까요 ... ^^

select IDX,TITLE,NAME,max(f1),max(f2),max(f3)
FROM
(
SELECT IDX,TITLE,NAME,
       CASE WHEN RN=1 THEN FILENAME ELSE '' END F1,
       CASE WHEN RN=2 THEN FILENAME ELSE '' END F2,
       CASE WHEN RN=3 THEN FILENAME ELSE '' END F3
FROM
(SELECT A.IDX,TITLE,NAME,FILENAME,row_number() OVER(PARTITION BY A.IDX ORDER BY FILENAME) RN
  FROM A A,B B
 WHERE A.IDX=B.IDX
) A1
) A2

group by idx,title,name
;