CREATE PROCEDURE ID2EAN13 (
    prefix varchar(10),
    id integer)
returns (
    barcode varchar(13))
as
declare variable n integer;
begin
      barcode = prefix || lpad(id, 12-char_length(prefix), '0');

      n = (cast (substring (barcode from  2 for 1) as integer)+
           cast (substring (barcode from  4 for 1) as integer)+
           cast (substring (barcode from  6 for 1) as integer)+
           cast (substring (barcode from  8 for 1) as integer)+
           cast (substring (barcode from 10 for 1) as integer)+
           cast (substring (barcode from 12 for 1) as integer)) * 3
           +
           cast (substring (barcode from  1 for 1) as integer)+
           cast (substring (barcode from  3 for 1) as integer)+
           cast (substring (barcode from  5 for 1) as integer)+
           cast (substring (barcode from  7 for 1) as integer)+
           cast (substring (barcode from  9 for 1) as integer)+
           cast (substring (barcode from 11 for 1) as integer);

      n = mod(n,10);
      if (n>0) then n = 10-n;
      barcode = barcode || cast (n as varchar(20));
  suspend;
end

-- Не понял, для чего тут было
      barcode = barcode || cast (n as varchar(20));

-- Если по логике должно быть так 
      barcode = barcode || cast (n as varchar(1));

-- А на самом деле достаточно так
      barcode = barcode || n;