create database xuexiao;
use xuexiao;

create table sc
(sno varchar(20),
cno varchar(20),
grade int);

create table course
(cno varchar(20),
cname varchar(20),
hours int);

create table student
(sno varchar(20),
sname varchar(20),
ssex char(10),
sage int,
sdept varchar(20));

 

insert into student
values("9512101","李1","男",19,"计算机系"),("9512102","刘晨","男",20,"计算机系"),
("9512103","王2","女",20,"计算机系"),("9512103","王敏","女",20,"计算机系"),
("9521101","张3","男",22,"信息系"),("9521102","吴宾","女",21,"信息系"),
("9521103","张4","男",20,"信息系"),("9531101","钱小力","女",18,"数学系"),
("9531102","王大","男",19,"数学系");

 

insert into course
values("c01","计算机文化学",70),("c02","VB",90),
("c03","计算机网络",80),("c04","数据库基础",108),
("c05","高等数学",180),("c06","数据结构",72);

insert into sc
values("9512101","c01",90),("9512101","c02",86),
("9512101","c06",null),("9512102","c02",78),
("9512102","c04",66),("9521102","c01",82),
("9521102","c02",75),("9521102","c04",92),
("9521102","c05",50),("9521103","c02",68),
("9521103","c06",null),("9531101","c01",80),
("9531101","c05",95),("9531102","c05",85);

 

#查询C01号课程成绩最高的分数

select max(grade) from sc where cno="c01";

 

#查询学生都选修了哪些课程,要求列出课程号

SELECT Cname AS 学生选修的课程,Cno AS 课程号
FROM course
WHERE Cno IN(SELECT DISTINCT Cno FROM SC); #DISTINCT用来去除重复

 

#统计每门课程的修课人数和考试最高分

SELECT course.cno,cname,COUNT(sno) AS 选课人数,MAX(grade) AS 最高分
FROM course
LEFT JOIN sc ON course.cno = sc.cno
GROUP BY sc.cno
ORDER BY course.cno;

 

#统计每个学生的选课门数,并按选课门数的递增顺序显示结果

select student.sno,sname,count(cno) as 选课门数 
from student 
left join sc on student.sno=sc.sno 
group by student.sno
order by 选课门数;

 

#查询选课门数超过2门的学生的平均成绩和选课门数

select student.sno,sname,avg(grade) as 平均成绩,count(cno) as 选课门数 
from student 
left join sc on student.sno=sc.sno
group by student.sno
having 选课门数>2;

 

#查询成绩80分以上的学生的姓名、课程号和成绩,并按成绩的降序排列结果

SELECT Sname 学生姓名,SC.Cno 课程号,SC.Grade 成绩
FROM student
left join SC ON student.Sno=SC.Sno #左(内)连接表SC查询
WHERE SC.Grade>80
ORDER BY SC.Grade DESC;

 

#分别查询信息系和计算机系的学生的姓名、性别、修课名称、修课成绩,
#并要求将这两个查询结果合并成一个结果集,
#并以系名、姓名、性别、修课名称、修课成绩的顺序显示各列

SELECT Sdept 系名,Sname 姓名,Ssex 性别,course.Cname 修课名称,SC.Grade 修课成绩
FROM student 
inner join SC ON student.Sno=SC.Sno
inner join course ON course.Cno=SC.Cno
WHERE Sdept='信息系'
UNION 
SELECT Sdept 系名,Sname 姓名,Ssex 性别,course.Cname 修课名称,SC.Grade 修课成绩
FROM student 
inner join SC ON student.Sno=SC.Sno
inner join course ON course.Cno=SC.Cno
WHERE Sdept='计算机系';

select sdept,sname,ssex,course.cname,sc.grade from student
left join sc on student.sno=sc.sno
left join course on sc.cno=course.cno
where sdept in ("信息系","计算机系")
order by sdept;