Easy SQL Group-By question. (I'm new)

Answer 1

Instead of counting, why don't you try summing a couple of IIF/CASE statements? That is, using IIF, your query would be:

SELECT ClassId, COUNT(*) AS students,
       SUM(IIF(StatusId = 1, 1, 0)) as Actives,
       SUM(IIF(StatusId = 2, 1, 0)) as Inactives
FROM StudentsClasses
INNER JOIN Students ON StudentsClasses.StudentID = Students.StudentID
GROUP BY ClassId

Answer 2

This is ANSI SQL (should work as long as your SQL implementation is ANSI compatible):

SELECT 
 classID,
 COUNT(*) AS "Students",
 SUM(CASE statusID WHEN 1 THEN 1 ELSE 0 END) AS "Actives",
 SUM(CASE statusID WHEN 2 THEN 1 ELSE 0 END) AS "Inactives",
FROM StudentsClasses class
INNER JOIN Students stud ON class.studentID = stud.studentID
GROUP BY classID

Answer 3

A statement like this will give you a row for each class with a class ID and the number of students in it.

SELECT classID, COUNT(*) AS studentsInClass
    FROM students
    WHERE classID
    IN (SELECT DISTINCT classID FROM students)
    GROUP BY classID

The WHERE x IN (...) thing is something I just learned.

I'm not really sure about the other part. Do you want this all in one table? You could UNION it in, but it would be messy since the data isn't the same.