已有三张表
student表：

选课表SC:

课程表Course:

1. 查询每个学生的学号、姓名、课程代号及选课成绩，不允许有重复列

SELECT Student.SID,Student.Sname,SC.CID,SC.Scores
FROM Student INNER JOIN SC ON Student.SID=SC.SID

2. 查询每个学生的学号、姓名、课程名及选课成绩

SELECT Student.SID,Student.Sname,Course.Cname,SC.Scores
FROM Student INNER JOIN SC ON Student.SID=SC.SID INNER JOIN Course ON SC.CID=Course.CID

3. 查询哪些课程没人选

SELECT Course.Cname
FROM Course LEFT OUTER JOIN SC ON course.cid = sc.cid
WHERE Scores IS NULL

4. 查询男生中没选课的学号、姓名、性别

SELECT Student.SID,Sname,Sex
FROM Student LEFT OUTER JOIN SC ON Student.SID=SC.SID
WHERE Sex=‘男’ AND Scores IS NULL

5.查询每个男生的选课门数，包括没有选课的男生，结果按照选课门数递减排序。

SELECT Student.Sname,COUNT(SC.Scores) AS 选课门数
FROM Student LEFT OUTER JOIN SC ON Student.SID=SC.SID
WHERE Sex=‘男’
GROUP BY Sname
ORDER BY 选课门数 DESC