declare
ewage exception;
vwage teacher.wage%type;
begin
loop:
vwage:=&wage;
insert into teacher values(999,'wang','professor',to_date('01091990','ddmmyyyy'),1000,vwage,101);
if vwage<0 then raise ewage;
end if;
exception
when ewage then dbms_output.put_line('教师工资不能为负值');
goto loop;
rollback;
end;