mysql外键约束怎么写

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

CREATE TABLE customers (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(100) NOT NULL
);

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

-- 检查父表中是否存在对应的数据
SELECT * FROM parent_table WHERE id = ?;

-- 如果不存在，先插入或更新父表中的数据
INSERT INTO parent_table (column1, column2) VALUES (?, ?);

-- 设置级联删除
ALTER TABLE child_table
ADD CONSTRAINT fk_name
FOREIGN KEY (child_column)
REFERENCES parent_table(parent_column)
ON DELETE CASCADE;