problem with accessing MQTT database (mosquitto.db)

Hello all,
I need some help, I`m a beginner in this field.
As I`m trying to make a small & simple IoT demonstrator, I am using MQTT Broker/ client installed in Raspberry pi to receive DHT22 sensor data from Mbed Microcontroller board NucleoF401RE (MQTT client) via Wifi-interface using ESP8266.
At this point I was able to establish the connection and to get the sensor data at my Raspberry Pi command line, the MQTT broker is able to store messages for clients on a file called mosquitto.db. but the problem is that I was not able to access this file! I`m getting just an empty file.
Does anyone know what should I do to get the stored sensors data?
do I need extra software for this?
my next step is to visualize the sensor Data in IBM Bluemix!