现有一个学生选修课程的数据库School其中存放以下三个表：学生Student学号姓名性别年龄系别即Sno char 3 Sname char4 Ssex char 2 Sage int Sdept char6；课程Course课程号课程名任课教师即Cno char3 Cname char8 teacher char4；选修SC学号课程号分数即Sno char 3 Cno char 3 Grade

1. 查询男生的选课记录：

SELECT * FROM Student s, SC c WHERE s.Sno = c.Sno AND s.Ssex = '男'

2. 查询选修了操作系统的学生姓名及系名：

SELECT Sname, Sdept FROM Student s, Course c, SC sc WHERE s.Sno = sc.Sno AND c.Cno = sc.Cno AND c.Cname = '操作系统'

3. 查询张三同学选修的所有课程的名称及其成绩：

SELECT c.Cname, sc.Grade FROM Student s, Course c, SC sc WHERE s.Sno = sc.Sno AND c.Cno = sc.Cno AND s.Sname = '张三'

4. 查询计算机和数学专业学生所选修的课程详细信息，即课程号，课程名和任课教师：

SELECT c.Cno, c.Cname, c.teacher FROM Student s, Course c, SC sc WHERE s.Sno = sc.Sno AND c.Cno = sc.Cno AND (s.Sdept = '计算机' OR s.Sdept = '数学')

5. 查询女生所选修课程的平均成绩：

SELECT AVG(sc.Grade) FROM Student s, SC sc WHERE s.Sno = sc.Sno AND s.Ssex = '女