Its not SQL, but you can query the Activity Log for "Bytes Trans", and this
will list all the individual clients and how much data each one sent. Paste
this into Notepad, and save the .TXT file. Open the .TXT file in Excel
delimited by space, and sum the required column.

Paste the result into another Excel spreadsheet that maintains the amount of
data backed up per day.

Cheers, Chris