Please, read my comments to the question.
I'm not sure your requirements, but try this:
SELECT c.ID AS CourseID, CASE WHEN r.ID IS NULL THEN 'Not reserved' ELSE 'Reserved' END AS ReservationID
FROM Course AS c LEFT JOIN Reservation AS r ON c.ID = r.ID
You can achive similar effect with
COALESCE
function:
SELECT c.ID AS CourseID, COALESCE(r.ID, 'Not reserved') AS ReservationID
FROM Course AS c LEFT JOIN Reservation AS r ON c.ID = r.ID
Please, read these articles:
Visual Representation of SQL Joins[
^]
CASE (SQL)[
^]
COALESCE (SQL)[
^]