设置mysql字段唯一值

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

CREATE TABLE orders (
    order_id INT AUTO_INCREMENT PRIMARY KEY,
    user_id INT,
    product_id INT,
    UNIQUE (user_id, product_id)
);

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

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

ALTER TABLE users DROP INDEX username;

ALTER TABLE users ADD UNIQUE (email);