同时触发器使插入和更新变得困难

Question

我在 Oracle 中有 4 个表：hotel、tourist、stay、leave。 stay 表涉及游客入住酒店，leave 表存储游客离开酒店的日期信息。

CREATE TABLE hotel (
    id NUMBER(5),
    name VARCHAR2(50),
    tenants_amount NUMBER(3)
);

ALTER TABLE hotel ADD CONSTRAINT hotel_c1
CHECK(tenants_amount>=0 AND tenants_amount<=100);

CREATE TABLE tourist (
    id NUMBER(5),
    name VARCHAR2(50)
);

CREATE TABLE stay (
    tourist_id NUMBER(5),
    hotel_id NUMBER(5)
);

CREATE TABLE leave (
    departure_date DATE,
    hotel_id NUMBER(5),
    tourist_id NUMBER(5)
);

我有兴趣检查tenants_amount 上的插入或更新hotel 是否与表stay 的内容一致，因此我为hotel 编写了此触发器

CREATE OR REPLACE TRIGGER hotel_trg
BEFORE INSERT OR UPDATE ON hotel
FOR EACH ROW
DECLARE
    amount NUMBER(3);
BEGIN
    SELECT COUNT(tourist_id) INTO amount FROM stay WHERE hotel_id=:NEW.id GROUP BY hotel_id;
    IF :NEW.tenants_amount!=amount THEN
        RAISE_APPLICATION_ERROR(-20001, 'Specified tenants amount differs from the system records');
    END IF;
EXCEPTION
    WHEN NO_DATA_FOUND THEN
        IF :NEW.tenants_amount!=0 THEN
            RAISE_APPLICATION_ERROR(-20001, 'Specified tenants amount differs from the system records');
        END IF;
END;
/

我还写了第二个触发器，用于存储leave的信息和hotel的tenants_amount属性的管理。当对 stay 进行 DML 操作时会触发此触发器，因为它是代表此关系的表

CREATE OR REPLACE TRIGGER stay_trg
BEFORE INSERT OR UPDATE OR DELETE ON stay
FOR EACH ROW
DECLARE
    amount NUMBER(3);
BEGIN
    IF INSERTING THEN
        SELECT tenants_amount INTO amount FROM hotel WHERE id=:NEW.hotel_id;
        UPDATE hotel SET tenants_amount=amount+1 WHERE id=:NEW.hotel_id;
    END IF;
    IF UPDATING AND :NEW.hotel_id!=:OLD.hotel_id THEN
        SELECT tenants_amount INTO amount FROM hotel WHERE id=:OLD.hotel_id;
        UPDATE hotel SET tenants_amount=amount-1 WHERE id=:OLD.hotel_id;
        INSERT INTO leave VALUES (SYSDATE, :OLD.hotel_id, :OLD.tourist_id);
        SELECT tenants_amount INTO amount FROM hotel WHERE id=:NEW.hotel_id;
        UPDATE hotel SET tenants_amount=amount+1 WHERE id=:NEW.hotel_id;
    END IF;
    IF DELETING THEN
        SELECT tenants_amount INTO amount FROM hotel WHERE id=:OLD.hotel_id;
        UPDATE hotel SET tenants_amount=amount-1 WHERE id=:OLD.hotel_id;
        INSERT INTO leave VALUES (SYSDATE, :OLD.hotel_id, :OLD.tourist_id);
    END IF;
END;
/

最后，我尝试插入一些行：

INSERT INTO hotel VALUES (1,'Hotel 1',0);
INSERT INTO tourist VALUES (1, 'Tourist 1');
INSERT INTO stay VALUES (1, 1);

我从酒店触发器中得到错误：

ERROR at line 1:
ORA-20001: Specified tenants amount differs from the system records
ORA-06512: at "HOTEL_TRG", line 11
ORA-04088: error during execution of trigger 'HOTEL_TRG'
ORA-06512: at "STAY_TRG", line 6
ORA-04088: error during execution of trigger 'STAY_TRG'

会发生这种情况：当stay 的触发器被触发时，它会尝试将酒店的tenants_amount 增加到id=1，这是一个触发hotel 触发器的更新。 hotel 的触发器检查tenants_amount 是否与stay 的内容一致，但更改仍然不可见，也没有找到任何行。这意味着 tenants_amount 应该为 0，但 hotel 的更新将其设置为 1。

我想知道如何解决这个问题。

Answer 1

触发stay_trg AFTER INSERT 而不是BEFORE INSERT