select count("ID_FK"), "ID_FK", "NUM"  from TABLE1  where "ID_FK"=new."ID_FK" and "NUM"=new."NUM"
into :REC_COUNT, oldID_FK, oldNUM;

CREATE TRIGGER ORDERS_B_IU FOR ORDERS
ACTIVE BEFORE INSERT OR UPDATE POSITION  0 
AS
declare variable REC_COUNT smallint;
declare variable oldID_FK  integer;
declare variable oldNUM varchar( 15 );

begin
select count("ID_FK"), "ID_FK", "NUM"  from TABLE1  where "ID_FK"=new."ID_FK" and "NUM"=new."NUM"
into :REC_COUNT, oldID_FK, oldNUM;
if (inserting) and (REC_COUNT> 0 ) then exception EXIST_ID_FK_ORDERS;
if (updating) and (REC_COUNT= 1 ) and (....) then exception EXIST_ID_FK_ORDERS;
end

CREATE TRIGGER ORDERS_B_IU FOR ORDERS
ACTIVE ????? INSERT OR UPDATE POSITION  0 
AS
begin
  if (sqlcode=- 803 ) then exception EXIST_ID_FK_ORDERS;
end