Q08. Top 3 services at each outlet (a window function).   [9 row(s)]
outlet | rnk | Service_id | bills | charged
-------+-----+------------+-------+---------
O01    | 1   | S0001      | 596   | 32344000
O01    | 2   | S0002      | 155   | 7032000
O01    | 3   | S0003      | 132   | 3786000
O02    | 1   | S0001      | 495   | 20904000
O02    | 2   | S0002      | 180   | 8388000
O02    | 3   | S0003      | 67    | 2028000
O03    | 1   | S0001      | 277   | 14384000
O03    | 2   | S0002      | 70    | 2940000
O03    | 3   | S0008      | 61    | 2905000
