对于不能更新外键的情况,通常需要先删除该外键,然后再重新插入新的数据。如果已经将外键设置为CASCADE删除,则可以通过删除主表中的对应记录来删除相关的外键记录。以下是示例代码:
--创建示例表
CREATE TABLE orders (
id INT PRIMARY KEY,
customer_id INT,
order_date DATE,
FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE CASCADE
);
CREATE TABLE customers (
id INT PRIMARY KEY,
name VARCHAR(50)
);
--示例数据
INSERT INTO customers (id, name) VALUES (1, 'John');
INSERT INTO orders (id, customer_id, order_date) VALUES (1, 1, '2021-01-01');
--删除外键
ALTER TABLE orders DROP FOREIGN KEY orders_ibfk_1;
--更新外键字段
UPDATE orders SET customer_id = 2 WHERE id = 1;
--重新插入外键
ALTER TABLE orders ADD CONSTRAINT orders_fk_customer_id FOREIGN KEY (customer_id) REFERENCES customers(id) ON DELETE CASCADE;
在上面的示例中,先将外键删除,然后更新外键所对应的字段,最后重新添加外键并设置为CASCADE删除。