Only one (alternative 2)

10g friendly approach to store primary currencies. No virtual column used. Structure seems a lot like Tom Kyte suggested. Difference is that all country currencies are here populated in the same table and the materialized view replaced with a foreign key constraint.

create table country(country varchar2(2) constraint country_pk primary key
                   , primary_currency varchar(3) not null
                    )
;

create table currency(country references country
                    , currency varchar(3)
                    , constraint currency_pk primary key (currency,country)
                    )
;

alter table country
  add constraint primary_curr_in_country_curr
  foreign key(primary_currency,country)
  references currency(currency,country)
  deferrable initially deferred
;

Populating

insert into country (country,primary_currency) values ('US','USS');

insert into currency (country,currency) values ('US','USD');

insert into currency (country,currency) values ('US','USN');

insert into currency (country,currency) values ('US','USS');

commit;

insert into country (country,primary_currency) values ('FI','EUR');

insert into currency (country,currency) values ('FI','EUR');

insert into currency (country,currency) values ('FI','FIM');

commit;

Changing the default. Easier than before. Less code

update country set primary_currency='USD' where country='US';

commit;

select * from country;
country primary_currency
US      USD
FI      EUR

select * from currency;
country currency
FI      EUR
FI      FIM
US      USD
US      USN
US      USS

Testing the model:

update country set primary_currency='USD' where country='FI';

commit;

ORA-02091: transaction rolled back
ORA-02291: integrity constraint (RAFU.PRIMARY_CURR_IN_COUNTRY_CURR) violated -
parent key not found

insert into country (country, primary_currency) values ('SW','SEK');

commit;

ORA-02091: transaction rolled back
ORA-02291: integrity constraint (RAFU.PRIMARY_CURR_IN_COUNTRY_CURR) violated -
parent key not found

There is only one default currency per country, should be obvious.

Be sure to run unindex on your production model.