SQL query count from instances

Question

I have a table contains bus and student sign up information:

CREATE TABLE [dbo].[BusSignupInstance](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [StudentId] [int] NOT NULL,
    [BusId] [int] NOT NULL
) ON [PRIMARY]

Here are some sample data:

Id  StudentId   BusId
1   1   1
2   2   1
3   3   1
4   1   2
5   4   2
6   1   3
7   5   3

I want the total number by bus ID. In this case will be:

BusId    Num of Students
1      2 (student id 2, 3)
2      1 (student id 4)
3      2 (student id 1, 5)

The result is based on the order of sign up, if student 1 sign up three times, only the last time is effective.

Answer 1

SELECT  busId, COUNT(*)
FROM    (
        SELECT  busId, studentId,
                ROW_NUMBER() OVER (PARTITION BY studentId ORDER BY id DESC) rn
        FROM    busSignupInstance
        ) q
WHERE   rn = 1
GROUP BY
        busId