mysql创建表字段值唯一

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

ALTER TABLE users
ADD UNIQUE (email);

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

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

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

UPDATE users SET username = 'john_doe' WHERE email = 'john@example.com';

ALTER TABLE users DROP INDEX email;