Cannot use group by and over(partition by) in the same query?

Yes, you can, but you should be consistent regarding the grouping levels. That is, if your query is a GROUP BY query, then in an analytic function you can only use "detail" columns from the "non-analytic" part of your selected columns. Thus, you can use either the GROUP BY columns or the non-analytic aggregates, like this example:

select product_id, company, 
sum(members) as No_of_Members, 
sum(sum(members)) over(partition by company) as TotalMembership 
From Product_Membership 
Group by Product_ID, Company

Hope that helps

SELECT col_1, col_2, sum(Value) over(partition by col_1) as sum_value
    -- also try changing "col_1" to "col_2" in OVER
from myTable
GROUP BY col_2,col_1 

I found the solution.

I do not need to use OVER(PARTITION BY col_1) because it is already in the GROUP BY clause. Thus, the following query gives me the right answer:

SELECT col_1, col_2, sum(Value) as sum_value
from myTable GROUP BY col_1, col_2

since I am already grouping w.r.t col_1 and col_2.

Dave, thanks, I got the idea from your post.