How can I change the code to run the way I want? - sql-server

Question

I'm trying to solve the "Gradebook Challenge" from KhanAcademy but not in the platform but in SQL Management Studio. And I haven't obtain the same result as on the platform.
This is my code and the error that appears when I try to run it in SQL Management Studio. Yes, I've done the research and now I realise that is not posible use the GROUP BY clause with alias column, but I don't know what else do. So please, I really need a hand on this.
SELECT COUNT(*), Grade,
CASE
WHEN Grade > 90 THEN 'A'
WHEN Grade > 80 THEN 'B'
WHEN Grade > 70 THEN 'C'
ELSE 'F'
END AS LetterGrade
FROM StudentsGrades
GROUP BY Grade

Answer 1

Push the query containing the CASE statement into a subquery or Common Table Expression, eg
use tempdb
go
create table StudentsGrades(Id int identity primary key, StudentId int, Grade int);
insert into StudentsGrades(StudentId,Grade)
values (1,90),(2,99),(3,40),(5,88);
with q as
(
SELECT *,
CASE
WHEN Grade > 90 THEN 'A'
WHEN Grade > 80 THEN 'B'
WHEN Grade > 70 THEN 'C'
ELSE 'F'
END AS LetterGrade
FROM StudentsGrades
)
SELECT LetterGrade, Count(*) CountOfGrade
from q
group by LetterGrade
outputs
LetterGrade CountOfGrade
----------- ------------
A 1
B 2
F 1
(3 rows affected)

Answer 2

Just another option is CROSS APPLY. It allows you to stack calculations and reference the alias.
Example
Select LetterGrade
,CountofGrade = count(*)
From StudentsGrades A
Cross Apply ( values ( CASE WHEN Grade > 90 THEN 'A'
WHEN Grade > 80 THEN 'B'
WHEN Grade > 70 THEN 'C'
ELSE 'F'
END
) )B(LetterGrade)
Group By LetterGrade
Results
LetterGrade CountofGrade
A 1
B 2
F 1