After having read http://amarok.kde.org/wiki/Amarok_File_Tracking i wonder why Amarok would not provide a simple Export/import entry in the menubar : for each "unique hash" it would store the associated play counts and ratings in an xml file.
Actually the user who want to export ratings has to deal with the db, i think it's too complicated for most of users.

My idea is not so original, so i guess there is something wrong with it

Why do you want to export the db in the first place? So you can have the same statistics on two computers?

Nope.
I'd like to reset my DB to build a new one from scratch because i suspect it to contains obsolete data (albums i have deleted maybe some covers paths ??).

It seems to me that 90% of the database content consists in data fetched from the songs ID3 tags (artist, title, album fields).
But ratings are not in the tags, that's why it makes sense to me to provide something to save them.

Having this export function, i could do the following to clean my database without losing ratings :
- erase DB
- build DB
- import an xml ratings file, it would fill ratings field of songs which are both present in the DB and in the file