mysql怎么建外键约束

CREATE TABLE child_table (
    id INT PRIMARY KEY,
    parent_id INT,
    FOREIGN KEY (parent_id) REFERENCES parent_table(id)
);

-- 创建父表 users
CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(50) NOT NULL
);

-- 创建子表 orders
CREATE TABLE orders (
    id INT AUTO_INCREMENT PRIMARY KEY,
    user_id INT,
    amount DECIMAL(10, 2) NOT NULL,
    FOREIGN KEY (user_id) REFERENCES users(id)
);

-- 检查父表结构
DESCRIBE parent_table;

-- 尝试插入无效数据
INSERT INTO orders (user_id, amount) VALUES (999, 100.00); -- 这将失败，因为 user_id 999 在 users 表中不存在

CREATE TABLE orders (
    id INT AUTO_INCREMENT PRIMARY KEY,
    user_id INT,
    amount DECIMAL(10, 2) NOT NULL,
    FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
);