create table product_mst(product_id number,product_value number);

insert into product_mst values(1,200);
insert into product_mst values(2,400);
insert into product_mst values(3,600);

create table product_stk(product_id number,location number);

insert into product_stk values(1,2);
insert into product_stk values(1,15);
insert into product_stk values(2,1);

select a.product_id,a.location,b.product_value 
from product_stk a left join product_mst b 
on b.product_id = a.product_id;

PRODUCT_ID   LOCATION PRODUCT_VALUE
---------- ---------- -------------
         1         15           200
         1          2           200
         2          1           400

PRODUCT_ID   LOCATION PRODUCT_VALUE
---------- ---------- -------------
         1         15           200
         1          2             0
         2          1           400

select a.product_id,a.location,nvl(b.product_value,0) 
from
(select a.product_id,a.location,b.product_value 
from product_stk a left join 
product_mst b on b.product_id = a.product_id) a left join
(select a.product_id,min(a.location) location,product_value 
from product_stk a left join 
product_mst b on b.product_id = a.product_id group by a.product_id,product_value) b 
on
b.product_id =a.product_id and b.location = a.location
order by a.product_id,a.location


PRODUCT_ID   LOCATION NVL(B.PRODUCT_VALUE,0)
---------- ---------- ----------------------
         2          1                    400
         1          2                    200
         1         15                      0

select a.product_id,a.location,nvl(b.product_value,0) from
(
	select a.product_id,a.location,b.product_value 
	from product_stk a left join product_mst b on 
	b.product_id = a.product_id
) a left join
(
	select a.product_id,min(a.location) location,product_value 
	from product_stk a left join product_mst b on 
	b.product_id = a.product_id group by a.product_id,product_value
) b on
b.product_id =a.product_id and 
b.location = a.location
order by a.product_id,a.location


PRODUCT_ID   LOCATION NVL(B.PRODUCT_VALUE,0)
---------- ---------- ----------------------
         1          2                    200
         1         15                      0
         2          1                    400

SQL> with 
  2    data as (
  3      select a.product_id, a.location, b.product_value,
  4             row_number() over (partition by a.product_id order by a.location) rn
  5      from product_stk a, product_mst b 
  6      where b.product_id = a.product_id
  7    )
  8  select product_id, location,
  9         decode(rn,1,product_value,0) product_value
 10  from data
 11  /
PRODUCT_ID   LOCATION PRODUCT_VALUE
---------- ---------- -------------
         1          2           200
         1         15             0
         2          1           400

3 rows selected.