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SELECT S#
FROM SC AS X,SC AS Y
WHERE X.S#=Y.S# AND X.C#='C1' AND Y.C#='C2'
SELECT s#
FROM sc
WHERE c# = 'C1'
EXCEPT
SELECT s#
FROM sc
WHERE c# = 'C2'
select s# from sc as a where exists(select 1 from sc where S#=a.S# AND C#='C1')
and not exist(select 1 from sc where S#=a.S# AND C#='C2')