create table ORDERS 
(
    ID3_ORD  integer not null,
    ID3_CLT  integer not null,
    ORDNUM3  varchar( 7 )
);

unit fnstr;

interface

function SelOrdNum(ANum: PChar): smallint; cdecl;

implementation

uses SysUtils;

function SelOrdNum(ANum: PChar): smallint; cdecl;
begin
 try
  Result := StrToInt(Copy(string(ANum),  1 , Length(string(ANum)) -  4 ));
 except
  on Exception do  Result:=- 1 ;
 end;
end;

end.

declare external function SEL_ORD_NUM
    cstring( 7 )
    returns smallint free_it
    entry_point 'SelOrdNum' module_name 'mtg_udf';

CREATE PROCEDURE MAX_ORD (
    PID3_CLT INTEGER)
RETURNS (
    PNEW_ORDNUM SMALLINT)
AS
begin
 select max(SEL_ORD_NUM(ORDERS.ORDNUM3)) from ORDERS
 where ORDERS.ID3_CLT=:PID3_CLT into :PNEW_ORDNUM;
end

 select max(SEL_ORD_NUM(ORDERS.ORDNUM3)) from ORDERS
 where ORDERS.ID3_CLT= 4 ;

select max(cast (substring (ORDERS.ORDNUM3 from  1  for  3 ) as integer)) from ORDERS
 where ORDERS.ID3_CLT= 4 

CREATE PROCEDURE MAX_ORD (
    PID3_CLT INTEGER)
RETURNS (
    PNEW_ORDNUM SMALLINT)
AS
begin
 select max(cast (substring (ORDERS.ORDNUM3 from  1  for  3 ) as integer)) from ORDERS
 where ORDERS.ID3_CLT=:PID3_CLT into :PNEW_ORDNUM;
end