Erreur lors de l'utilisation sauf dans une requête

Je ne crois pas que MySQL supporte la syntaxe EXCEPT. Essayez d'utiliser NOT IN ou un LEFT JOIN :

SELECT s.sno 
FROM students s    
WHERE s.sno NOT IN 
(
    SELECT t.sno 
    FROM take t 
    WHERE t.cno = 'CS112'
);

OU

SELECT s.sno 
FROM students s    
    LEFT JOIN take t ON s.sno = t.sno
WHERE IFNULL(t.cno, '') != 'CS112'

MISE À JOUR

J'ai simulé vos données en tant que telles et elles renvoient correctement 5 à 10 :

create temporary table temp_students (sno int)

insert into temp_students values (1)
insert into temp_students values (2)
insert into temp_students values (3)
insert into temp_students values (4)
insert into temp_students values (5)
insert into temp_students values (6)
insert into temp_students values (7)
insert into temp_students values (8)
insert into temp_students values (9)
insert into temp_students values (10)

create temporary table temp_take (sno int, cno varchar(50))

insert into temp_take values (1, 'CS112')
insert into temp_take values (2, 'CS112')
insert into temp_take values (3, 'CS112')
insert into temp_take values (4, 'CS112')

SELECT s.sno 
FROM temp_students s    
        LEFT JOIN temp_take t ON s.sno = t.sno
WHERE IFNULL(t.cno, '') != 'CS112'