查询选修了课程的学生姓名。

以下是两种查询选修了课程的学生姓名的SQL语句：

1.使用NOT EXISTS子查询：

SELECT Sname
FROM Student
WHERE NOT EXISTS(
    SELECT *
    FROM Course
    WHERE NOT EXISTS(
        SELECT *
        FROM SC
        WHERE SC.Sno = Student.Sno AND SC.Cno = Course.Cno
    )
);

2.使用COUNT(*)和GROUP BY子句：

SELECT Sname
FROM Student
WHERE Sno IN (
    SELECT Sno
    FROM SC
    GROUP BY Sno
    HAVING COUNT(*) = (
        SELECT COUNT(*)
        FROM Course
    )
);