mysql创建表unique

-- 创建单列唯一约束
CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) UNIQUE,
    email VARCHAR(100) UNIQUE
);

-- 创建多列唯一约束
CREATE TABLE orders (
    order_id INT AUTO_INCREMENT PRIMARY KEY,
    user_id INT,
    order_number VARCHAR(50),
    UNIQUE (user_id, order_number)
);

-- 检查是否存在重复数据
SELECT * FROM users WHERE username = 'existing_username';

-- 如果存在重复数据，可以选择删除或更新
DELETE FROM users WHERE username = 'existing_username';

-- 或者更新为新的唯一值
UPDATE users SET username = 'new_unique_username' WHERE id = 1;

SHOW CREATE TABLE users;

ALTER TABLE users DROP INDEX username;