mysql 如何创建外键约束

ALTER TABLE child_table
ADD CONSTRAINT fk_name
FOREIGN KEY (child_column)
REFERENCES parent_table(parent_column);

CREATE TABLE customers (
    id INT PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(255) NOT NULL
);

CREATE TABLE orders (
    id INT PRIMARY KEY AUTO_INCREMENT,
    order_date DATE NOT NULL,
    customer_id INT,
    FOREIGN KEY (customer_id) REFERENCES customers(id)
);

-- 示例：确保父表中有对应的值
INSERT INTO customers (name) VALUES ('John Doe');
INSERT INTO orders (order_date, customer_id) VALUES ('2023-10-01', 1);

ALTER TABLE orders
DROP FOREIGN KEY fk_name;