SQL Query Problem: How Many Students have one or more blood donations in 2016?

I am a high school student and am currently working on an exercise about SQL, which is this query that made me stuck for an hour and still couldn’t solve it. The table schema is as follows:

Table

The question my teacher gave me was: How any Students have one or more blood donations in 2016?
Thanks for any generous help. The SQL I tried to run is

SELECT COUNT(*)   
  FROM DONA        
 WHERE StudID IN (SELECT StudID
                    FROM DONA
                   WHERE YEAR(DonDate)= 2016
                   GROUP BY StudID
                  HAVING COUNT(*)>1)
 GROUP BY StudID

And I still couldn’t figure out how to COUNT the number of students.

Answer

You can join both the tables and use the keyword DISTINCT to find DISTINCT students-

SELECT COUNT(StudID)
  FROM (SELECT DISTINCT StudID
          FROM DONA D
         WHERE YEAR(DonDate) = 2016) T;