我正在做一个将船租给客户的项目，并根据他们输入的信息为他们提供船。我想在此项目的一个部分中创建一个触发器，该触发器基于你何时乘船离开还是何时返回，将计算出天数，然后将钱退还给客户余额或收取额外费用。我有这样的触发器设置-

CREATE OR REPLACE TRIGGER BALANCE_FEE
AFTER INSERT ON CHARTERS
FOR EACH ROW 
DECLARE
FEE NUMBER;
ACL_DATE DATE;
EXP_DATE DATE;
GRP_ID NUMBER;

BEGIN

SELECT ACL_RETURN_DATE, EXP_RETURN_DATE, GRP_ID INTO ACL_DATE, EXP_DATE, GRP_ID FROM CHARTERS;


IF ACL_DATE > EXP_DATE
THEN
FEE := (ACL_DATE - EXP_DATE) * 75;

IF ACL_DATE < EXP_DATE
THEN
FEE := (ACL_DATE - EXP_DATE)* -20;

ELSE
FEE := 0;
END IF;
END IF;
UPDATE CUSTOMER
     SET CUSTOMER.BALANCE = CUSTOMER.BALANCE + FEE
     WHERE CUSTOMER.GRP_ID = GRP_ID;
END;
/
SHOW ERROR;

如果它也有帮助，那么这里有两个与此触发器相关的表。

CREATE TABLE CUSTOMER (
    CUS_FNAME VARCHAR(20),
    CUS_LNAME VARCHAR(20),
    GENDER VARCHAR(20),
    PHONENUM NUMBER(10),
    CITY VARCHAR(20),
    PARTY_COUNT INT,
    GRP_ID VARCHAR(20) PRIMARY KEY,
    BALANCE NUMBER);

CREATE TABLE CHARTERS (
    CHARTER_ID VARCHAR(20),
    BOAT_ID VARCHAR(20) REFERENCES BOAT(BOAT_ID),
    GRP_ID VARCHAR(20) REFERENCES CUSTOMER(GRP_ID),
    EXP_RETURN_DATE DATE,
    ACL_RETURN_DATE DATE);

当我运行代码时，我得到此错误报告

INSERT INTO CHARTERS (CHARTER_ID,BOAT_ID,EXP_RETURN_DATE,ACL_RETURN_DATE,GRP_ID) VALUES ('T003','B003',DATE '2019-05-5',DATE '2019-05-07','G003')
Error report -
ORA-04091: table ADMIN_BF.CHARTERS is mutating, trigger/function may not see it
ORA-06512: at "ADMIN_BF.BALANCE_FEE", line 9
ORA-04088: error during execution of trigger 'ADMIN_BF.BALANCE_FEE'

这到底是什么问题？这与我的触发器是“插入后”有关吗？提前致谢。