mysql unique的用法

-- 创建表时添加UNIQUE约束
CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100) NOT NULL UNIQUE
);

-- 插入数据
INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com');
-- 尝试插入重复的email
INSERT INTO users (username, email) VALUES ('jane_doe', 'john@example.com'); -- 这将导致错误

ERROR 1062 (23000): Duplicate entry 'john@example.com' for key 'email'

-- 更新现有记录
UPDATE users SET username = 'jane_doe' WHERE email = 'john@example.com';

ALTER TABLE users DROP INDEX email;