MYSQL 1215 无法添加外部约束

Question

所以我不知道为什么我不能在表 ORDER_LINE 中添加外部约束，我确保所有类型都是正确的。请帮忙，它一直给我错误

Error Code: 1215. Cannot add foreign key constraint

CREATE TABLE IF NOT EXISTS Customer (
    Customer_ID int NOT NULL AUTO_INCREMENT,
    Customer_Name VARCHAR(50) NOT NULL,
    Customer_Age INT UNIQUE,
    Customer_Address VARCHAR(255),
    Customer_City VARCHAR(255),
    Customer_State VARCHAR(50),
    Customer_Zip VARCHAR(20),
    PRIMARY KEY(Customer_ID)
);

CREATE TABLE IF NOT EXISTS Sales_order (
    Order_ID int AUTO_INCREMENT,
    Order_date DATE,
    Customer_ID int,
    PRIMARY KEY(Order_ID),
    FOREIGN KEY(Customer_ID) REFERENCES Customer(Customer_ID) ON DELETE     
    CASCADE
);

CREATE TABLE IF NOT EXISTS Products (
    Product_ID int AUTO_INCREMENT,
    Product_Description VARCHAR(255),
    Product_Finish VARCHAR(50),
    Standard_Price DECIMAL,
    Product_Line_ID INT,
    PRIMARY KEY(Product_ID)
);

CREATE TABLE IF NOT EXISTS ORDER_LINE   (
    Order_ID int,
    Product_ID int,
    Ordered_Quantity int,
    PRIMARY KEY(Order_ID, Product_ID),
    FOREIGN KEY(Order_ID) REFERENCES Sales_order(Order_ID) ON DELETE SET NULL,
    FOREIGN KEY(Product_ID) REFERENCES Products(Product_ID)
);

Answer 1

这可能是因为 primary key columns are made NOT NULL ，因此您的 ON DELETE SET NULL 正在尝试设置无效值。

编辑:事实上，我刚刚测试过，这似乎是问题所在——如果我删除 SET NULL 或主键，它会创建表。