mysql怎么让数据表内容不能重复

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 (username);
ALTER TABLE users ADD UNIQUE (email);

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50),
    email VARCHAR(100)
);

CREATE UNIQUE INDEX idx_unique_username ON users(username);
CREATE UNIQUE INDEX idx_unique_email ON users(email);

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

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

DELIMITER //
CREATE TRIGGER check_unique_username
BEFORE INSERT ON users
FOR EACH ROW
BEGIN
    IF EXISTS (SELECT 1 FROM users WHERE username = NEW.username) THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Username already exists';
    END IF;
END//
DELIMITER ;