插入或删除后的Oracle触发器

Question

对不起我的英语不好。我有2张桌子：Table1idtable2_idnummodification_date 和Table2idtable2num我想打一个触发器，它插入或删除操作后在Table1更新的最后一个值num的Table2.table1lastnum。我的触发器：CREATE OR REPLACE TRIGGER TABLE1_NUM_TRG  AFTER INSERT OR DELETE ON table1  FOR EACH ROWBEGIN  IF INSERTING then  UPDATE table2  SET    table2num = :new.num  WHERE  table2.id = :new.table2_id;  ELSE  UPDATE table2  SET    table2num = (SELECT num FROM  (SELECT num FROM table1 WHERE table2_id = :old.table2_id ORDER BY modification_date DESC) WHERE ROWNUM <= 1)  WHERE  table2.id = :old.table2_id;  END IF;END TABLE1_NUM_TRG; 但删除后Table1我有错误：ORA-04091: table BD.TABLE1 is mutating, trigger/function may not see itORA-06512: at "BD.TABLE1_NUM_TRG", line 11ORA-04088: error during execution of trigger 'BD.TABLE1_NUM_TRG'我究竟做错了什么？