Джек,

Я обычно деляю так (для любой таблицы):

CREATE GENERATOR MyTable_GEN_ID;

CREATE OR ALTER TRIGGER MyTable_BI0 FOR MyTable
ACTIVE BEFORE INSERT POSITION 0
AS
declare variable CurrentCode integer;
begin
if ((NEW.MyTable_ID is NULL) or (NEW.MyTable_ID<=0)) then
NEW.MyTable_ID = GEN_ID(MyTable_GEN_ID,1);
else begin
CurrentCode = GEN_ID(MyTable_GEN_ID,0);
if (CurrentCode<NEW.MyTable_ID) then
CurrentCode = GEN_ID(MyTable_GEN_ID,NEW.MyTable_ID-CurrentCode);
end
end