Feedback form with rating system [closed]

I want to create a Feedback form for my website with rating system, which will then display the monthly average rating to a separate page as a graph.

I do have a little concept. But It's lacking something. I know how to create a feedback form. But I am confused on how to save it in Mysql. I mean do I need to calculate the average rating before saving it to mysql or when calling from the graphs page.

Please help me with concept or example.

Thanks.