Oracle 表正在变异，触发器可能看不到它

创建触发器时没有错误。但是当我启动它时，一条错误消息显示表正在发生变化。我如何解决这个表变异的问题

CREATE OR REPLACE TRIGGER tgr_invoice
AFTER UPDATE OF discharge_date
ON Admission
FOR EACH ROW
DECLARE
doc_fee NUMBER(5);
nights_stayed NUMBER(4);
room_price NUMBER(5);
room_fee NUMBER(9);
med_fee NUMBER(7, 2);
units_prescribed NUMBER(3);
price_per_unit NUMBER(7, 2);
total_fee NUMBER(8,2);
BEGIN
SELECT consult_fee INTO doc_fee     
FROM Doctor
WHERE staff_id IN (SELECT seen_by FROM Admission
WHERE patient_id = :NEW.patient_id
AND admission_date = :NEW.admission_date);

SELECT (discharge_date - admission_date) INTO nights_stayed 
FROM Admission 
WHERE patient_id = :NEW.patient_id
AND admission_date = :NEW.admission_date;

SELECT room_price INTO room_price 
FROM Room
WHERE room_no IN (SELECT room_no FROM Admission
WHERE patient_id = :NEW.patient_id
AND admission_date = :NEW.admission_date);

SELECT units_prescribed INTO units_prescribed 
FROM Prescription
WHERE patient_id = :NEW.patient_id
AND prescription_date = :NEW.admission_date;

SELECT price_per_unit INTO price_per_unit 
FROM Medicine
WHERE med_id IN (SELECT med_id FROM Prescription 
WHERE patient_id = :NEW.patient_id
AND prescription_date = :NEW.Admission_date);

room_fee := room_price * nights_stayed;
med_fee := units_prescribed * price_per_unit;
total_fee := room_fee + doc_fee + med_fee;

INSERT INTO Invoice VALUES(seq_inv_no.NEXTVAL, :NEW.patient_id, 
:NEW.admission_date, doc_fee, room_fee, med_fee, total_fee);
END;
/

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当我更新入院表并输入出院日期时，出现了一个错误“表系统。入院正在发生变化，触发器/函数可能看不到它“

我认为更合适的做法是，在创建DB触发器的表列的：新值或：旧值之间进行操作，从而从赋值中派生值，而不是使用SELECT语句作为行级触发器。因此，考虑使用下面的代码块：

CREATE OR REPLACE TRIGGER tgr_invoice AFTER UPDATE OF discharge_date ON Admission
FOR EACH ROW
DECLARE
    doc_fee NUMBER(5);
    nights_stayed NUMBER(4);
    room_price NUMBER(5);
    room_fee NUMBER(9);
    med_fee NUMBER(7, 2);
    units_prescribed NUMBER(3);
    price_per_unit NUMBER(7, 2);
    total_fee NUMBER(8,2);
BEGIN
 SELECT consult_fee 
   INTO doc_fee     
   FROM Doctor
  WHERE staff_id = :NEW.seen_by;

 nights_stayed := :NEW.discharge_date - :NEW.admission_date; 

 SELECT room_price 
   INTO room_price 
   FROM Room
  WHERE room_no = :NEW.room_no;

 SELECT units_prescribed 
   INTO units_prescribed 
   FROM Prescription
  WHERE patient_id = :NEW.patient_id
    AND prescription_date = :NEW.admission_date;

 SELECT price_per_unit INTO price_per_unit 
   FROM Medicine
  WHERE med_id IN (SELECT med_id 
                     FROM Prescription 
                    WHERE patient_id = :NEW.patient_id
                      AND prescription_date = :NEW.Admission_date);

 room_fee := room_price * nights_stayed;
 med_fee := units_prescribed * price_per_unit;
 total_fee := room_fee + doc_fee + med_fee;

 INSERT INTO Invoice 
 VALUES(seq_inv_no.NEXTVAL, :NEW.patient_id, :NEW.admission_date, 
        doc_fee, room_fee, med_fee, total_fee);
END;
/     

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如果您描述了您试图实现的逻辑，这会有所帮助。示例数据和所需结果有帮助。您不应在

系统

（或任何其他Oracle系统架构）中创建任何用户表或其他对象。为你的应用程序创建一个用户并在那里创建你的对象。非常感谢你的帮助，我非常感谢。