mysql属性唯一性

-- 创建表并添加单列唯一性约束
CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) UNIQUE,
    email VARCHAR(100) UNIQUE
);

-- 创建表并添加多列唯一性约束
CREATE TABLE orders (
    id INT AUTO_INCREMENT PRIMARY KEY,
    order_number VARCHAR(50),
    customer_id INT,
    UNIQUE (order_number, customer_id)
);

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

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

SHOW CREATE TABLE users;

ALTER TABLE users DROP INDEX username;