You could also do it in a single query using a join if UNION doesn't count for "single query":
SELECT s2.* FROM Semester2 s2 LEFT OUTER JOIN Semester1 s1 ON s2.StudentId = s1.StudentId AND s2.SubjectId = s1.SubjectId WHERE s1.StudentId IS NULL;
The WHERE clause will make it so only results where there isn't a perfect match in Semester1 appear.
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