MySQL以数据库user_db及数据表student、teacher、class、course、sc为基础（11）查询选修“数据库原理”的学生的最高成绩、最低成绩、平均成绩； （12）查询平均成绩在60分以上的课程及其平均成绩； （13）查询平均成绩大于“高莉”的平均成绩的学生学号、姓名及平均成绩； （14）查询每门课程中成绩最低的学生的学号、姓名、性别、院系； （15）查询只被一名学生选修的课程的课程号、课程名。

11.

SELECT MAX(score) AS max_score, MIN(score) AS min_score, AVG(score) AS avg_score 
FROM sc 
WHERE course_name = '数据库原理';

12.

SELECT course_id, course_name, AVG(score) AS avg_score 
FROM sc 
GROUP BY course_id, course_name 
HAVING AVG(score) > 60;

13.

SELECT s.stu_id, s.stu_name, AVG(score) AS avg_score 
FROM student s, sc c 
WHERE s.stu_id = c.stu_id 
GROUP BY s.stu_id, s.stu_name 
HAVING AVG(score) > (SELECT AVG(score) FROM sc WHERE stu_name = '高莉');

14.

SELECT s.stu_id, s.stu_name, s.gender, s.department, c.course_name, MIN(score) AS min_score 
FROM student s, sc c 
WHERE s.stu_id = c.stu_id 
AND score = (SELECT MIN(score) FROM sc WHERE course_name = c.course_name)
GROUP BY c.course_name, s.stu_id, s.stu_name, s.gender, s.department;

15.

SELECT course_id, course_name 
FROM sc 
GROUP BY course_id, course_name 
HAVING COUNT(DISTINCT stu_id) = 1;