mysql给字段添加唯一约束

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,
    product_id INT NOT NULL,
    UNIQUE (user_id, product_id)
);

-- 检查是否存在重复数据
SELECT username, COUNT(*)
FROM users
GROUP BY username
HAVING COUNT(*) > 1;

-- 删除重复数据
DELETE FROM users
WHERE id NOT IN (
    SELECT MIN(id)
    FROM users
    GROUP BY username
);

-- 再次添加唯一约束
ALTER TABLE users ADD UNIQUE (username);

-- 检查插入的数据是否唯一
SELECT * FROM users WHERE username = 'new_username';

-- 如果数据已存在，可以选择更新或放弃插入
INSERT INTO users (username, email)
VALUES ('new_username', 'new_email')
ON DUPLICATE KEY UPDATE email = 'new_email';