mysql创建外键的语句

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),
    email VARCHAR(100)
);

CREATE TABLE orders (
    id INT AUTO_INCREMENT PRIMARY KEY,
    order_date DATE,
    amount DECIMAL(10, 2),
    customer_id INT
);

ALTER TABLE orders
ADD CONSTRAINT fk_customer
FOREIGN KEY (customer_id)
REFERENCES customers(id);

ALTER TABLE orders
ADD CONSTRAINT fk_customer
FOREIGN KEY (customer_id)
REFERENCES customers(id)
ON DELETE SET NULL;

ALTER TABLE orders
ADD CONSTRAINT fk_customer
FOREIGN KEY (customer_id)
REFERENCES customers(id)
ON DELETE CASCADE;