create procedure change_priority(id integer, priority integer)
as
declare variable old integer;
declare variable pr integer;
begin
   select priority from table_name where id=:id into :old;
   if old is null then begin
     UPDATE table_name SET Priority=Priority+ 1  
     WHERE Priority>=:priority
     exit;
   end
   if priority is null then begin
     UPDATE table_name SET Priority=Priority- 1  
     WHERE Priority>=:old
     exit;
   end
   if old<priority then begin
     for select priority from table_name 
          where priority between :old+ 1  and :priority+ 1 
          into :pr 
     do begin
       update table_name set priority=priority- 1  where priority=:pr
     end
     exit;
   end
   if old>priority then begin
     for select priority from table_name 
          where priority between :old and :priority
          into :pr 
     do begin
       update table_name set priority=priority+ 1  where priority=:pr
     end
     exit;
   end
end