CREATE TABLE IF NOT EXISTS expenses (
    id INT AUTO_INCREMENT PRIMARY KEY,
    driver_id INT,
    vehicle_id INT,
    expense_date DATE NOT NULL,
    category VARCHAR(255) NOT NULL,
    amount DECIMAL(10, 2) NOT NULL,
    description TEXT,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    FOREIGN KEY (driver_id) REFERENCES drivers(id) ON DELETE SET NULL,
    FOREIGN KEY (vehicle_id) REFERENCES vehicles(id) ON DELETE SET NULL
);