@erwin-brandstetter Please, benchmark this:
SELECT s.stud_id, s.name
FROM student s, student_club x, student_club y
WHERE x.club_id = 30
AND s.stud_id = x.stud_id
AND y.club_id = 50
AND s.stud_id = y.stud_id;
It's like number 6) by @sean , just cleaner, I guess.