The following is an incomplete SQL query that finds the sids of suppliers who supply red parts but not green parts:

SELECT DISTINCT C$.$sid

FROM Catalog C$,$ Parts P

WHERE C$.$pid $=$ P$.$pid AND P$.$color $=$Red$$

$?$

SELECT DISTINCT C$1.$sid

FROM Catalog C$1,$ Parts P$1$

WHERE C$1.$pid $=$ P$1.$pid AND P$1.$color $=$Green$$

The missing operator $($at $?)$ is: