CREATE TABLE INVOICES (
  ID_INVOICE VARCHAR(30) CHARACTER SET NONE NOT NULL,
  ID_SUPPLIER VARCHAR(3) CHARACTER SET NONE,
  INV_NBR VARCHAR(30) CHARACTER SET NONE NOT NULL,
  INV_DATE DATE,
  DUE_DATE DATE,
  PAYM_DELAY COMPUTED BY (f_ageindays(inv_date, due_date))
);

select
  count( invoices.id_supplier ) count_of_id_supplier,
  invoices.paym_delay,
  invoices.id_supplier
from invoices
  where
  (
    (invoices.inv_date >= :user_date)
  )
group by invoices.paym_delay, invoices.id_supplier 

with max_derived as (
  select max(count_of_id_supplier), id_supplier
  from inv_derived
  group by id_supplier 
) 
select 
  paym_delay,
  id_supplier
from inv_derived
inner join on (id_supplier = max_derived.id_supplier and count_of_id_supplier = max_derived.count_of_id_supplier )