1。触发器不能从触发器所对应的基表中读取数据;
2。create or replace trigger TR_INV
       after update of ZPRICE on INV for each row
　  begin
       if :old.ZPRICE>:new.ZPRICE then
         raise_application_error(-20001,''工资只涨不降'');
       end if;
　 end;
3。instead-of 触发器用于在复杂视图上设置触发器
    1）。必须指定 for each row;
    2）。只适用视图;
    3）。不能有 before /after;
    4）。视图不能指定 with check option;
    5）。demo：
        view:
          create or replace view V_EMP_DEPT
          as
          select
            E.ZEMP_ID,
            E.ZDESC ZEMP_DESC,
            D.ZDEPT_ID,
            D.ZDESC ZDEPT_DESC
          from EMP E
          left join DEPT D on D.ZDEPT_ID=E.ZDEPT_ID;
         
        trigger:
          create or replace trigger TR_EMP_DEPD
            instead of insert on V_EMP_DEPT for each row
          declare
            DEPT_COUNT int;
          begin
            select count(*) into DEPT_COUNT from DEPT where ZDEPT_ID=:new.ZDEPT_ID
            if DEPT_COUNT=0 then
              insert into DEPT values(:new.ZEMP_ID,:new.ZDEPT_ID,:new.ZDEPT_DESC);
            end if;
           
            select count(*) into DEPT_COUNT from EMP where ZEMP_ID=:new.ZEMP_ID
            if DEPT_COUNT=0 then
              insert into EMP values(:new.ZEMP_ID,:new.ZEMP_DESC,:new.ZDEPT_ID);
            end if;           
          end;    
4。重新编译触发器：当alter table 了以后，需要重新编译一次该表上的触发器：
          alter trigger TR_EMP compile;