Problem of Order By Clause

Question

I have a student marksheet table. like that..

 

SELECT * FROM MARKSHEET_TAB ORDER BY FULL_MARKS DESC

 

Reg_No	Marks_Obtain	Full_Marks	Roll_No
R1051	651	900	22
R1032	645	900	5
R1013	645	900	3
R1025	638	900	8
R1011	625	900	12

 

 I want to display these records if those students have obtained same marks then rank sheet will generated based on their class roll no. that is..

 

Reg_No	Marks_Obtain	Full_Marks	Roll_No	Rank
R1051	651	900	22	1
R1013	645	900	3	2
R1032	645	900	5	3
R1025	638	900	8	4
R1011	625	900	12	5

Please solve this problem.