Update with group by in subquery

create table a1 (id varchar2(20), sal number(3));

insert into a1('a',1);

insert into a1('b',2);

insert into a1('c',3);

create table a2 (id varchar2(20), sal number(3));

insert into a2('a',1);

insert into a2('a',5);

insert into a2('b',8);

Now we've to update a1.sal with sum(b1.sal) like

update a1 set sel=select sal from(

select id,sum(sal)sal from a2 group by id) b

where a1.id=b.id;

how can we do that...?