Day 35 作业(sql查询)

1. 查询所有大于60分的学生的姓名和学号 (DISTINCT: 去重)

select distinct student.sid, sname from score join student on student_id = student.sid where score.number > 60;

-- 2.查询每个老师教授的课程数量 和 老师信息

select teacher.tid, teacher.tname, count(cid) as course_num from teacher join course on teacher.tid = course.teach_id group by teacher.tid, teacher.tname;

-- 3. 查询学生的信息以及学生所在的班级信息

select * from student join class  on student.class_id = class.cid;

-- 4、学生中男生的个数和女生的个数

select gender, count(*) as num from student group by gender;

-- 5、获取所有学习'生物'的学生的学号和成绩；姓名

select student.sid, student.sname, score.number from student join score on student.sid = score.student_id join course on score.course_id = course.cid where course.cname = '生物';

-- 6、查询平均成绩大于60分的同学的学号和平均成绩；

select student.sid, avg(number) as avg_number from student join score on student.sid = score.student_id group by student.sid having avg_number > 60;

-- 7、查询姓“李”的老师的个数；

select count(tname) from teacher where tname like '李%'

-- 8、查询课程成绩小于60分的同学的学号、姓名；

select student.sid, sname from score join student on student_id = student.sid where score.number < 60;

-- 9. 删除学习“叶平”老师课的SC表记录

delete from student where student.sid in (select score.student_id from course, teacher, score where course.teach_id = teacher.tid and course.cid = score.course_id and teacher.tname = '叶平');
//有外键连接的数据不允许删除

-- 10.查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分；

select course.cid, max(score.number) as max_num, min(score.number) as min_num from course join score on course.cid = score.course_id group by course.cid;

-- 11.查询每门课程被选修的学生数

select course.cname,count(sname) as choose_num from course join score on course.cid = score.course_id join student on score.student_id = student.sid group by course.cname;

-- 12.查询姓“张”的学生名单；

select * from student where sname like '张%'

-- 13.查询每门课程的平均成绩，结果按平均成绩升序排列，平均成绩相同时，按课程号降序排列

select course.cid, course.cname, avg(score.number) as avg_score_number from course join score on course.cid = score.course_id group by course.cid, course.cname order by avg_score_number, course.cid desc;

-- 14.查询平均成绩大于85的所有学生的学号、姓名和平均成绩

select student.sid, student.sname, avg(score.number) as avg_score_number from student join score on student.sid = score.student_id group by student.sid, student.sname having avg_score_number > 85;

-- 15.查询课程编号为3且课程成绩在80分以上的学生的学号和姓名；

select student.sid, student.sname from student join score on student.sid = score.student_id where score.course_id = 3 and score.number >= 80;

-- 16.查询各个课程及相应的选修人数

select course.cname, count(student_id) as student_num from course, score where course.cid = score.course_id group by course.cname

-- 17.查询“4”课程分数小于60，按分数降序排列的同学学号

select student_id from score where course_id = 4 and number < 60 order by number desc;

-- 18.删除学号为“2”的同学的“1”课程的成绩

delete from score where student_id = 2 and course_id = 1