mysql中设置列为唯一

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100) NOT NULL UNIQUE
);

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)
);

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

INSERT INTO users (username, email) VALUES ('user1', 'example@example.com')
ON DUPLICATE KEY UPDATE username=VALUES(username);

DELETE FROM users WHERE email = 'example@example.com' LIMIT 1;
INSERT INTO users (username, email) VALUES ('user1', 'example@example.com');