Sum without using 2 selects

Question

How can I get this result without using 2 queries to sum buys and sells?
BANK  TOTAL_SHARES
BANK1           30
BANK2           25
BANK4           20
BANK5           20
BANK3           15
Script to test
create table test (BUYER varchar2(5), SELLER varchar2(5), shares numeric(10));
insert into test values ('BANK1','BANK1',10);
insert into test values ('BANK1','BANK2',10);
insert into test values ('BANK3','BANK2',15);
insert into test values ('BANK4','BANK5',20);
commit;
select bank, sum(shares) total_shares from
select buyer bank, sum(shares) shares from test group by buyer
union
select seller bank, sum(shares) shares from test group by seller
group by bank
order by total_shares desc, bank;

Answer 1

So .. you have 2 records for BANK1:
insert into test values ('BANK1','BANK1',10);
insert into test values ('BANK1','BANK2',10);
You want to add that first 10 , twice .. and the 2nd 10 once?
BANK1           30
So you want to double add some of those?