Update column values by other column values

Question

How to write a query which will update column A on table A with the data from table B ? Table A and table B are linked by ID. ID is a primary key of table A and foreign key of table B. I need to summarize the amount column in table b by id, and then update the sum column in table A by ID.

QUERY

create table A
(Id number,
SumAmount number
);

insert into A 
SELECT 1, NULL FROM DUAL
UNION All
SELECT 2, NULL FROM DUAL
UNION ALL
SELECT 3, NULL FROM DUAL
UNION All
SELECT 4, NULL FROM DUAL
UNION ALL
SELECT 5, NULL FROM DUAL
UNION All
SELECT 6, NULL FROM DUAL


create table B
(Id number,
Amount number
);


insert into B
SELECT 1, 100 FROM DUAL
UNION All
SELECT 1, 200 FROM DUAL
UNION ALL
SELECT 1, 320 FROM DUAL
UNION All
SELECT 2, 230 FROM DUAL
UNION ALL
SELECT 4, 246 FROM DUAL
UNION All
SELECT 4, 212 FROM DUAL

I've wrote a query for this but it doesn't work. Any helps are appreciated.

update a
set a.sumamount = (
select sum(b.amount)
from b group by b.id
where a.id = b.id
)

Answer 1

Your query is alright, if you swap group by and where :

update a
set a.sumamount = (
select sum(b.amount)
from b
where a.id = b.id
group by b.id
);

Example at SQL Fiddle.

Answer 2

update A set 
SumAmount = (
select sum(B.Amount)
from B group by B.Id
HAVING (A.Id=B.Id) )