Divide by Zero Encountered with PARTITION BY

I have multiple subqueries for groups of students (a,b,c) and I want to display the results by subject. Some groups may not be in certain subjects.

I'm receiving a 'divide by zero' error when performing a calculation. How can I modify the code below to display 'null' when the result is n/a?

STR(((100.0*COUNT(a.STC_grade))/SUM(COUNT(a.STC_grade))OVER(partition by dbo.S85_STUDENT_ACAD_CRED.stc_subject)), 5,0)  +'%' AS 'PERCENT'

Any assistance would be appreciated.

If you added input data, your query and what your expectation were, it would be easies to recreate your situation. Not sure that my example fully covers your input data but I could get your error and fix it by adding isnull in SUM(COUNT(isnull(sg.STC_grade,0)))

create table Subjects (Sbj_Id int)
create table Groups (Grp_Id int)
create table Sub_Grp (Grp_id int, Sbj_Id int, STC int, STC_grade int)

insert into Subjects
values
(1),
(2),
(3)

insert into Groups
values
(1),
(2)

insert into Sub_Grp
values
(1,1,1,5),
(1,2,1,15),
(1,2,2,30),
(1,2,3,10),
(2,2,1,20),
(2,3,1,40)

select g.Grp_Id as [Group], s.Sbj_Id as [Subject], sg.STC as [Student], sg.STC_grade
from Groups g
cross apply Subjects s
left join Sub_Grp sg on sg.Grp_Id = g.Grp_id and sg.Sbj_Id = s.Sbj_Id

Highlighted rows from the screen above will ruin the query below without isnull.

select g.Grp_Id as [Group], s.Sbj_Id as [Subject],
STR(((100.0*COUNT(sg.STC_grade))/SUM(COUNT(isnull(sg.STC_grade,0)))OVER(partition by sg.Sbj_Id)), 5,0)  +'%' AS 'PERCENT'
from Groups g
cross apply Subjects s
left join Sub_Grp sg on sg.Grp_Id = g.Grp_id and sg.Sbj_Id = s.Sbj_Id
group by g.Grp_Id,s.Sbj_Id,sg.Sbj_Id
order by s.Sbj_Id, g.Grp_Id