CREATE TABLE IF NOT EXISTS user_favorites (
    id INT AUTO_INCREMENT PRIMARY KEY,
    user_id INT NOT NULL,
    drill_id INT DEFAULT NULL,
    training_session_id INT DEFAULT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE,
    FOREIGN KEY (drill_id) REFERENCES drills(id) ON DELETE CASCADE,
    FOREIGN KEY (training_session_id) REFERENCES training_sessions(id) ON DELETE CASCADE,
    UNIQUE KEY user_favorite_item (user_id, drill_id, training_session_id)
);