This will perform a global update of all db rows, adding 10%:
"UPDATE table_name SET money=money+money/100*10";
The money column must be a FLOAT type.
I couldn't tell you offhand how mysql rounds numbers which exceed the maximum permitted float length: ie (when 5 is the first out of range float integer) always up, always down, or up when previous float integer is even and down when odd? That kind of precision might not matter to you anyway. See mysql.com for manual and further info on FLOAT columns.
You might also want to set a date value somewhere in the add interest script and check that it is over a year since the last execution of the script: this would ensure that annual interest isn't added more often than it should be by mistake.
