mysql外键关联

-- 检查关联表中是否存在对应的数据
SELECT * FROM related_table WHERE id = 123;

-- 如果不存在，需要先插入或更新关联表中的数据
INSERT INTO related_table (id, name) VALUES (123, 'example');

-- 检查关联表中是否存在对应的数据
SELECT * FROM related_table WHERE id = 123;

-- 如果不存在，需要先插入或更新关联表中的数据
UPDATE related_table SET name = 'new_name' WHERE id = 123;

-- 检查关联表中是否存在对该数据的引用
SELECT * FROM referencing_table WHERE foreign_key_column = 123;

-- 如果存在引用，需要先删除或更新关联表中的引用
DELETE FROM referencing_table WHERE foreign_key_column = 123;

-- 创建customers表
CREATE TABLE customers (
    id INT PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(255) NOT NULL
);

-- 创建orders表
CREATE TABLE orders (
    id INT PRIMARY KEY AUTO_INCREMENT,
    customer_id INT,
    order_date DATE,
    FOREIGN KEY (customer_id) REFERENCES customers(id)
);

-- 插入数据
INSERT INTO customers (name) VALUES ('Alice');
INSERT INTO orders (customer_id, order_date) VALUES (1, '2023-01-01');

-- 查询数据
SELECT * FROM customers;
SELECT * FROM orders;