CREATE TABLE users (
    user_id VARCHAR(20) PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    password VARCHAR(100) NOT NULL,
    role ENUM('student', 'teacher', 'admin') NOT NULL,
    email VARCHAR(100),
    phone VARCHAR(20),
    status ENUM('active', 'frozen') DEFAULT 'active',
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);