looping between the colums on of every row each cells

Question

Dear,

 

I have table in sql server like this

 

 

studentid	name	marks1	marks2	marks3	marks4	marks5	marks6	marks7	Total Sub	Need_Percentage of Marks
1	Sanjeev	78	87	76	78	95			5	?
2	Sandeep	78	87	76	78	95	59		6	?
3	Sumit	78			78	95	52		4	?
4	Sudhir	78	85	0	78	95	52		6	?
5	Ram	AB	85	23	85	12			5	?

 

i want the percentage of total marks obtained by the students

 

when i am fetching the data using sqldatareader it shows me the dbnull error because it is not getting the value kindly tell the solution any one..

 

how to loop  between the coloums

 

here blank cell means not having value...........

i cannot assign any cell value 0 because it also come in marks it take the null then there is conversion issue so kindly tell me the solution as soon as possible.........

 

Regards

Sanjeev