create function max_open_orders() returns trigger as $max_open_orders$
        declare
                num_orders   record;
        begin
               select into num_orders count(o_orderkey) as num from orders where o_orderstatus='O' group by o_custkey having o_custkey=NEW.o_custkey;
                if (num_orders.num = 18) then
                  raise exception '% cannot have more than 18 open orders', NEW.custkey;
                end if;
                return null;
        end;
$max_open_orders$ language plpgsql;