mysql建表唯一性约束

-- 创建单列唯一性约束
CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100) NOT NULL UNIQUE
);

-- 创建多列唯一性约束
CREATE TABLE orders (
    order_id INT AUTO_INCREMENT PRIMARY KEY,
    user_id INT,
    order_number VARCHAR(50) NOT NULL,
    UNIQUE (user_id, order_number)
);

-- 使用INSERT IGNORE
INSERT IGNORE INTO users (username, email) VALUES ('john_doe', 'john@example.com');

-- 使用ON DUPLICATE KEY UPDATE
INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com')
ON DUPLICATE KEY UPDATE email = 'john@example.com';

SHOW CREATE TABLE users;

ALTER TABLE users DROP INDEX username;
ALTER TABLE orders DROP INDEX user_id_order_number;