T-SQL SELECT Using CASE and String Concatenation

Here I'm selecting the ID field followed by a concatenation of 3 different subject fields, depending on whether or not the fields are NULL.

SELECT
  ID, 
  CASE 
  WHEN Subject2 IS NULL THEN 
    Subject1
  WHEN Subject2 IS NOT NULL AND Subject3 IS NULL THEN 
    Subject1 +  ' : ' + Subject2
  ELSE Subject1 + ' : '  + Subject2 + ' : '  + Subject3
  END AS TheSubject
FROM Subjects
ORDER BY TheSubject