Oracle：ON DELETE CASCADE 导致触发器递归

Question

我们以这两个表为例：Customer 包含客户，Product 包含客户购买/使用的产品。

每个产品都通过外键 CustomerID 引用一个客户，该外键对应于表 Customer 的主键（它们也具有相同的名称）。

删除客户时，所有引用该客户的产品都将被删除：Product.CustomerID 具有属性ON DELETE CASCADE。

现在假设基地的客户至少应该拥有一种产品：
当一个产品被移除时，如果它是一个客户的最后一个产品，那么这个客户也必须被移除。

CREATE OR REPLACE TRIGGER RemoveCustomer
AFTER DELETE ON Product
BEGIN
        DELETE FROM Customer
        WHERE CustomerID IN (
                SELECT c.CustomerID
                FROM Customer c
                LEFT OUTER JOIN Product p
                    ON p.CustomerID = c.CustomerID
                GROUP BY c.CustomerID HAVING COUNT(p.CustomerID) = 0
        );
END;
/

这个解决方案对我来说似乎很自然，但 Oracle 不喜欢它。 每次删除产品时，我都会收到错误消息：

ORA-00036: maximum number of recursive SQL levels (50) exceeded 

这即使 DELETE 不会导致程序被删除。

令人惊讶的是，这种语法工作得很好：

CREATE OR REPLACE TRIGGER RemoveCustomer
AFTER DELETE ON Product
BEGIN
        FOR my_row IN (
                SELECT c.CustomerID
                FROM Customer c
                LEFT OUTER JOIN Product p
                    ON p.CustomerID = c.CustomerID
                GROUP BY c.CustomerID HAVING COUNT(p.CustomerID) = 0
        ) 
        LOOP
                DELETE FROM Customer WHERE CustomerID = my_row.CustomerID;
        END LOOP;

END;
/

有人能解释一下为什么会这样吗？

编辑：

这里有一个工作示例：

CREATE TABLE Customer (
    CustomerID INTEGER                 PRIMARY KEY
);

CREATE TABLE Product (
    ProductID INTEGER                 PRIMARY KEY,
    CustomerID INTEGER,
    CONSTRAINT fk_Customer FOREIGN KEY (CustomerID)
                                        REFERENCES Customer
                                        ON DELETE CASCADE
);


INSERT INTo Customer (CustomerID) VALUES (0);
INSERT INTo Customer (CustomerID) VALUES (1);
INSERT INTo Customer (CustomerID) VALUES (2);
INSERT INTo Customer (CustomerID) VALUES (3);
INSERT INTo Customer (CustomerID) VALUES (4);
INSERT INTo Customer (CustomerID) VALUES (5);
INSERT INTo Customer (CustomerID) VALUES (6);

INSERT INTO Product (ProductID, CustomerID) VALUES (0, 0);
INSERT INTO Product (ProductID, CustomerID) VALUES (1, 0);
INSERT INTO Product (ProductID, CustomerID) VALUES (2, 1);
INSERT INTO Product (ProductID, CustomerID) VALUES (3, 2);
INSERT INTO Product (ProductID, CustomerID) VALUES (4, 3);
INSERT INTO Product (ProductID, CustomerID) VALUES (5, 3);
INSERT INTO Product (ProductID, CustomerID) VALUES (6, 3);
INSERT INTO Product (ProductID, CustomerID) VALUES (7, 4);
INSERT INTO Product (ProductID, CustomerID) VALUES (8, 5);
INSERT INTO Product (ProductID, CustomerID) VALUES (9, 5);
INSERT INTO Product (ProductID, CustomerID) VALUES (10, 6);


CREATE OR REPLACE TRIGGER RemoveCustomer
AFTER DELETE ON Product
BEGIN
        DELETE FROM Customer
        WHERE CustomerID IN (
                SELECT c.CustomerID
                FROM Customer c
                LEFT OUTER JOIN Product p
                    ON p.CustomerID = c.CustomerID
                GROUP BY c.CustomerID HAVING COUNT(p.CustomerID) = 0
        );
END;
/


/* This request will produce the error */
DELETE FROM Product WHERE CustomerID = 3;

Answer 1

令人惊讶，但似乎products 上的级联删除语句总是 在对customers 执行删除之后发生 - 即使没有删除客户。例如：

SQL> delete customer where customerid = 9999999;
delete customer where customerid = 9999999
       *
ERROR at line 1:
ORA-00036: maximum number of recursive SQL levels (50) exceeded
ORA-06512: at "TTEST.REMOVECUSTOMER", line 2
...

使用您的触发器的第二个版本，for 循环体在没有客户没有产品时永远不会执行，因此customers 的删除永远不会发生，并且可以避免无限循环。