You are going to join the subquery back to the main table, right? Otherwise it'll always return all the rows in the products table!

Anyway, you could do something like:

- Use count(*) over (partition by <join cols>) to see how many rows there are in products for the account
- Only return rows from the subquery where this count = 1:

create table t1 (
  x int,
  y int
);
create table t2 (
  x int,
  z int
);

insert into t1 values (1, 1);
insert into t1 values (1, 2);
insert into t1 values (2, 2);

insert into t2 values (1, 0);
insert into t2 values (2, 0);
           
update t2 
set    z = (select y
            from   (select x, y, count(*) over (partition by x) c from t1) t1 
            where  t1.x = t2.x
            and    c = 1
            );
            
select * from t2;

X  Z  
1     
2  2