数据库StudentCourse 有表SC含Sno作学号,Cno作课程号，Grade作成绩。表Student含Sno作学号,Sname作姓名，Ssex作性别，Sbirthday作出生日期，Sdept作所在系，Memo作备注。表Course含Cno作课程号，Cname作姓名，PreCourse作先行课,PreCno作选修课程号，Credit作学分，Semester作开课学期。①查询选修了【数据库原理】的计算机系的学生学号和姓名。②查询每一门课的间接先行课(印先行课的先行课)。③查询学生的学号、姓名，选修课程的名称和成绩。④查询选修了课程的学生姓名。⑤查询所有学生的信息和所选修的课程的课程号。⑥查询已被选修的课程的情况和所有课程的名字。⑦列出学生所有可能的选修情况。⑧查找计算机系的学生选修课程数大于2的学生的姓名、平均成绩和选课门数，并按平均成绩降序排列：

①查询选修了【数据库原理】的计算机系的学生学号和姓名。

SELECT Student.Sno, Student.Sname
FROM Student, SC, Course
WHERE Student.Sno = SC.Sno AND SC.Cno = Course.Cno AND Course.Cname = '数据库原理' AND Student.Sdept = '计算机系';

②查询每一门课的间接先行课(印先行课的先行课)。

WITH RECURSIVE pre_course(cno, pre_cno) AS (
    SELECT Cno, PreCno FROM Course WHERE PreCno IS NOT NULL
    UNION ALL
    SELECT pre_course.cno, Course.PreCno FROM pre_course JOIN Course ON pre_course.pre_cno = Course.Cno
)
SELECT cno, group_concat(DISTINCT pre_cno) as indirect_pre_courses
FROM pre_course
GROUP BY cno;

③查询学生的学号、姓名，选修课程的名称和成绩。

SELECT Student.Sno, Student.Sname, Course.Cname, SC.Grade
FROM Student, SC, Course
WHERE Student.Sno = SC.Sno AND SC.Cno = Course.Cno;

④查询选修了课程的学生姓名。

SELECT DISTINCT Student.Sname
FROM Student, SC, Course
WHERE Student.Sno = SC.Sno AND SC.Cno = Course.Cno AND Course.Cname = '数据库原理';

⑤查询所有学生的信息和所选修的课程的课程号。

SELECT Student.*, SC.Cno
FROM Student LEFT JOIN SC ON Student.Sno = SC.Sno;

⑥查询已被选修的课程的情况和所有课程的名字。

SELECT Course.*, SC.Sno, SC.Grade
FROM Course LEFT JOIN SC ON Course.Cno = SC.Cno;

⑦列出学生所有可能的选修情况。

SELECT Student.Sno, Student.Sname, Course.Cno, Course.Cname
FROM Student CROSS JOIN Course;

⑧查找计算机系的学生选修课程数大于2的学生的姓名、平均成绩和选课门数，并按平均成绩降序排列。

SELECT Student.Sname, AVG(SC.Grade) AS avg_grade, COUNT(*) as course_count
FROM Student JOIN SC ON Student.Sno = SC.Sno JOIN Course ON SC.Cno = Course.Cno
WHERE Student.Sdept = '计算机系'
GROUP BY Student.Sno
HAVING COUNT(*) > 2
ORDER BY AVG(SC.Grade) DESC;