Update Data To A Table From The Sum Of A Field From Another Table Based On Some Criteria

Hello Friends,

I have two tables, And also I have Sample data in them.

create table X
(y int,
m int,
v int)

insert into X select 2007,1,5
insert into X select 2007,1,3
insert into X select 2007,2,9
insert into X select 2007,2,1

select * from X

Create table Y
(fy int,
fm int,
v int)

insert into Y select 2007,1,0
insert into Y select 2007,2,0
insert into Y select 2007,3,0

select * from X
select * from Y

I want to update the Table Y with the Sum of the Fields V from X based on the Criteria Y.fy = X.y and Y.fm = X.m

Using temporary table cannot be done.

Thanks in Advance,
Babz