How to calculate percentage with a SQL statement

Question

1. The most efficient (using over()).

   select Grade, count(*) * 100.0 / sum(count(*)) over()
   from MyTable
   group by Grade
   

2. Universal (any SQL version).

   select Grade, count(*) * 100.0 / (select count(*) from MyTable)
   from MyTable
   group by Grade;
   

3. With CTE, the least efficient.

   with t(Grade, GradeCount) 
   as 
   ( 
       select Grade, count(*) 
       from MyTable
       group by Grade
   )
   select Grade, GradeCount * 100.0/(select sum(GradeCount) from t)
   from t;
   

I have tested the following and this does work. The answer by gordyii was close but had the multiplication of 100 in the wrong place and had some missing parenthesis.

Select Grade, (Count(Grade)* 100 / (Select Count(*) From MyTable)) as Score
From MyTable
Group By Grade

Instead of using a separate CTE to get the total, you can use a window function without the "partition by" clause.

If you are using:

count(*)

to get the count for a group, you can use:

sum(count(*)) over ()

to get the total count.

For example:

select Grade, 100. * count(*) / sum(count(*)) over ()
from table
group by Grade;

It tends to be faster in my experience, but I think it might internally use a temp table in some cases (I've seen "Worktable" when running with "set statistics io on").

EDIT: I'm not sure if my example query is what you are looking for, I was just illustrating how the windowing functions work.

I simply use this when ever I need to work out a percentage..

ROUND(CAST((Numerator * 100.0 / Denominator) AS FLOAT), 2) AS Percentage

Note that 100.0 returns 1 decimal, whereas 100 on it's own will round up the result to the nearest whole number, even with the ROUND(...,2) function!