如何设计一个可维护的MySQL表结构来实现在线预约功能？

CREATE TABLE users (
    user_id INT PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(255) NOT NULL,
    phone_number VARCHAR(20) NOT NULL,
    email VARCHAR(255) NOT NULL,
    passWord VARCHAR(255) NOT NULL
);

CREATE TABLE appointments (
    appointment_id INT PRIMARY KEY AUTO_INCREMENT,
    user_id INT NOT NULL,
    appointment_date DATE NOT NULL,
    appointment_time TIME NOT NULL,
    location VARCHAR(255) NOT NULL,
    FOREIGN KEY (user_id) REFERENCES users(user_id)
);

CREATE TABLE user_appointment_mapping (
    user_id INT NOT NULL,
    appointment_id INT NOT NULL,
    PRIMARY KEY (user_id, appointment_id),
    FOREIGN KEY (user_id) REFERENCES users(user_id),
    FOREIGN KEY (appointment_id) REFERENCES appointments(appointment_id)
);

ALTER TABLE appointments ADD UNIQUE (appointment_date, appointment_time);