语法的MySQL问题-外键

-- 检查引用的主键是否存在
SELECT * FROM referenced_table WHERE primary_key = 'value_to_check';

-- 确保插入的数据在引用的表中存在
INSERT INTO referencing_table (foreign_key_column) VALUES ('existing_primary_key_value');

-- 使用CASCADE选项，删除主键记录时自动删除外键记录
ALTER TABLE referencing_table ADD CONSTRAINT fk_name FOREIGN KEY (foreign_key_column) REFERENCES referenced_table(primary_key) ON DELETE CASCADE;

-- 或者先删除外键记录，再删除主键记录
DELETE FROM referencing_table WHERE foreign_key_column = 'value_to_delete';
DELETE FROM referenced_table WHERE primary_key = 'value_to_delete';

-- 优化查询语句，减少不必要的连接操作
SELECT * FROM referencing_table WHERE foreign_key_column IN (SELECT primary_key FROM referenced_table WHERE condition);

-- 考虑使用物化视图（Materialized View）来预先计算和存储连接结果

-- 创建 users 表
CREATE TABLE users (
    user_id INT PRIMARY KEY,
    username VARCHAR(50) NOT NULL
);

-- 创建 orders 表，并添加外键约束
CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    user_id INT,
    order_date DATE,
    FOREIGN KEY (user_id) REFERENCES users(user_id)
);

-- 插入数据
INSERT INTO users (user_id, username) VALUES (1, 'Alice');
INSERT INTO orders (order_id, user_id, order_date) VALUES (101, 1, '2023-04-01');

-- 查询关联数据
SELECT * FROM orders JOIN users ON orders.user_id = users.user_id;