Hi there, I feel confused about why here they use except and why WHERE C.sid = S.sid is here as a condition? Thanks!

3. Find the snames of suppliers who supply every red part.

SELECT S.sname

FROM Suppliers S

WHERE NOT EXISTS (( SELECT P.pid

FROM Parts P

WHERE P.color = 'red' )

EXCEPT

( SELECT C.pid

FROM Catalog C

WHERE C.sid = S.sid ));