mysql确定唯一性

-- 创建表并添加单列唯一约束
CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100) NOT NULL UNIQUE
);

-- 创建表并添加复合唯一约束
CREATE TABLE orders (
    order_id INT AUTO_INCREMENT PRIMARY KEY,
    user_id INT NOT NULL,
    order_date DATE NOT NULL,
    UNIQUE (user_id, order_date)
);

INSERT IGNORE INTO users (username, email) VALUES ('john_doe', 'john@example.com');

INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com')
ON DUPLICATE KEY UPDATE email = 'john@example.com';