How to sort products by number of sales.

[PanMS]

Hello.

Does anybody know how to add to "sort by" function on product list page one more type of sorting:

sort by number of sales.

For example if the product was sold 100 times its the first in the list, if it was sold 0 times it is the last in the list.

 

By this time I became a bit familiar with CSS and TPL coding but it seems to me that it needs to have some php coding.

So if it's simple coding? please tell me which file I have to edit and how.

Or if anybody can give me some advice. I will also appreciate it very much.