SELECT s.name AS "Symptoom"
FROM ziekte z
INNER JOIN koppeling k ON k.ziekte_id = z.id
INNER JOIN symptomen s ON s.id =k.symptoom_id
WHERE z.name = "mazelen"

EXCEPT

SELECT s.name AS "Symptoom"
FROM ziekte z
INNER JOIN koppeling k ON k.ziekte_id = z.id
INNER JOIN symptomen s ON s.id =k.symptoom_id
WHERE z.name = "diabetes"