Using UNION sub query inside of NOT IN MySQL

Question

I want to use a union sub query inside not null in MySQL. The query is shown below.

SELECT s.student_id
     , s.student_name
  FROM students` s
 WHERE s.student_id NOT IN (
    SELECT student_id
      FROM free_students
     UNION
    SELECT student_id
    FROM paid_students
)

If the UNION sub query returns null the query results also becomes null. How to handle these kind of situations?

Answer 1

I would use an exists clause, two of them actually:

SELECT s.student_id, s.student_name
FROM students s
WHERE NOT EXISTS (SELECT 1 FROM free_students fs WHERE s.student_id = fs.student_id) AND
      NOT EXISTS (SELECT 1 FROM paid_students ps WHERE s.student_id = ps.student_id);