mysql中唯一约束条件

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

-- 创建表并设置多列唯一约束
CREATE TABLE orders (
    order_id INT AUTO_INCREMENT PRIMARY KEY,
    user_id INT NOT NULL,
    order_number VARCHAR(50) NOT NULL,
    UNIQUE (user_id, order_number)
);

-- 检查是否存在重复数据
SELECT * FROM users WHERE username = 'existing_user';

-- 更新数据
UPDATE users SET email = 'new_email@example.com' WHERE username = 'existing_user';

-- 删除重复数据
DELETE FROM users WHERE username = 'duplicate_user';

-- 查看表的唯一约束
SHOW INDEX FROM users;