mysql如何创建外键

CREATE TABLE child_table (
    column1 datatype,
    column2 datatype,
    ...
    FOREIGN KEY (child_column) REFERENCES parent_table(parent_column)
);

-- 创建客户表
CREATE TABLE customers (
    customer_id INT PRIMARY KEY AUTO_INCREMENT,
    customer_name VARCHAR(255) NOT NULL
);

-- 创建订单表，并添加外键约束
CREATE TABLE orders (
    order_id INT PRIMARY KEY AUTO_INCREMENT,
    order_date DATE NOT NULL,
    customer_id INT,
    FOREIGN KEY (customer_id) REFERENCES customers(customer_id)
);

-- 确保customer_id列有索引
ALTER TABLE customers ADD INDEX idx_customer_id (customer_id);

ALTER TABLE orders DROP FOREIGN KEY fk_customer_id;