How to find ranking

Question

hi..
i have a database table just like below.
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cliencode  InvoiceNo  datetime
1  1001  10/10/2011
1  1002   10/10/2011
1  1003   10/10/2011
2  1004   10/10/2011
2  1005   10/10/2011
2  1006   10/10/2011
3  1007   10/10/2011
1  1008   10/10/2011
1  1009   10/10/2011
2  1010   10/10/2011
___________________________________

now i want to find ranking of client code so how it is possible.
please help