How to make the first match in a group a different group

Question

I need to group by a certain column and to make the first occurrence into another group.

select * from table;

id	currency	amount
1	USD	1
2	USD	2
3	EUR	3
4	IL	4
5	EUR	5
6	USD	6

So something like that:

select min(id) id, currency, sum(amount)
from table
group by currency, 
         case when min(id)=id then 'first-occurrence' else 'rest' end;

id	currency	amount
1	USD	1
2	USD	8
3	EUR	3
4	IL	4
5	EUR	5

Of course the case doesn't make much sense. The goal is make it also fast, and if possible not to read the table again as the table is huge.

Answer 1

RANK helped me:

select min(id) id, currency, sum(amount)
FROM (select id, currency, amount, 
              RANK () OVER
                   ( PARTITION BY currency
                     ORDER BY id 
                   ) a_rank
      from table)
group by currency, 
         case when a_rank=1 then 'first-occurrence' 
                            else 'rest' 
         end;