mysql字段加唯一约束

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

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

INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com'); -- 会报错，因为用户名已存在

-- 删除重复数据
DELETE FROM users WHERE username = 'john_doe' AND email = 'john@example.com' LIMIT 1;

-- 再次插入数据
INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com');