# Fonctions

```sql
insert into exchange_rate values
  ('EUR', '1999-01-04', 1),
  ('USD', '1999-01-04', 1.1789),
  ('YEN', '1999-01-04', 133.73);
```

```sql
create or replace function latest_exchange_rate (
  p_code text,
  p_date date
)
returns decimal
language sql
as $$
select rate
  from exchange_rate
  where currency_code = p_code
    and date < p_date
    order by date desc
    limit 1;
$$;
```