mysql 两个字段唯一约束

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)
);

INSERT INTO users (username, email)
SELECT 'newuser', 'newuser@example.com'
FROM DUAL
WHERE NOT EXISTS (
    SELECT 1 FROM users WHERE username = 'newuser'
);

ALTER TABLE users DROP INDEX username;
ALTER TABLE users DROP INDEX email;