mysql 唯一约束unique

-- 创建表并添加单列唯一约束
CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100) NOT NULL UNIQUE
);

-- 创建表并添加多列唯一约束
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)
);

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

-- 如果存在重复数据，可以选择更新或删除
UPDATE users SET email = 'new_email@example.com' WHERE username = 'existing_user';

-- 或者删除重复数据
DELETE FROM users WHERE username = 'existing_user';

-- 使用 INSERT IGNORE 忽略唯一约束错误
INSERT IGNORE INTO users (username, email) VALUES ('existing_user', 'new_email@example.com');

-- 删除单列唯一约束
ALTER TABLE users DROP INDEX username;

-- 删除多列唯一约束
ALTER TABLE orders DROP INDEX user_id_order_number;