mysql 创建字段Unique

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

-- 创建多列唯一约束
CREATE TABLE orders (
    order_id INT AUTO_INCREMENT PRIMARY KEY,
    customer_id INT NOT NULL,
    product_id INT NOT NULL,
    UNIQUE (customer_id, product_id)
);

-- 检查是否存在重复值
SELECT username, email FROM users WHERE username = 'existing_username' OR email = 'existing_email';

-- 删除重复值
DELETE FROM users WHERE username = 'existing_username' OR email = 'existing_email';

-- 或者更新重复值
UPDATE users SET username = CONCAT(username, '_duplicate') WHERE username = 'existing_username';

-- 使用INSERT IGNORE忽略重复值
INSERT IGNORE INTO users (username, email) VALUES ('new_username', 'new_email');

-- 或者使用ON DUPLICATE KEY UPDATE更新重复值
INSERT INTO users (username, email) VALUES ('new_username', 'new_email')
ON DUPLICATE KEY UPDATE username = 'new_username', email = 'new_email';