创建四张表：学生表Student，课程表Course，教师表Teacher，分数表SC
1.学生表Student(id,name,age,sex):
create table Student(id varchar(10),name varchar(10),age date,sex varchar(10));
insert into Student values
('01' , '赵雷' , '1990-01-01' , '男'),
('02' , '钱电' , '1990-12-21' , '男'),
('03' , '孙风' , '1990-12-20' , '男'),
('04' , '李云' , '1990-12-06' , '男'),
('05' , '周梅' , '1991-12-01' , '女'),
('06' , '吴兰' , '1992-01-01' , '女'),
('07' , '郑竹' , '1989-01-01' , '女'),
('08' , '张三' , '2017-12-20' , '女'),
('09' , '李四' , '2017-12-25' , '女'),
('10' , '李四' , '2012-06-06' , '女'),
('11' , '赵六' , '2013-06-13' , '女'),
('12' , '孙七' , '2014-06-01' , '女');
2.课程表Course(id,name,tid)
create table Course(id varchar(10),name varchar(10),tid varchar(10));
insert into Course values
('01' , '语文' , '02'),
('02' , '数学' , '01'),
('03' , '英语' , '03');
3.教师表Teacher(id,name)
create table Teacher(id varchar(10),name varchar(10));
insert into Teacher values('01' , '张三'),('02' , '李四'),('03' , '王五');
4.分数表SC(sid,cid,score)
create table SC(sid varchar(10),cid varchar(10),score decimal(18,1));
insert into SC values('01' , '01' , 80),('01' , '02' , 90),
('01' , '03' , 99),('02' , '01' , 70),('02' , '02' , 60),('02' , '03' , 80),
('03' , '01' , 80),('03' , '02' , 80),('03' , '03' , 80),('04' , '01' , 50),
('04' , '02' , 30),('04' , '03' , 20),('05' , '01' , 76),('05' , '02' , 87),
('06' , '01' , 31),('06' , '03' , 34),('07' , '02' , 89),('07' , '03' , 98);
练习题10道：
1.查询" 01 "课程比" 02 "课程成绩高的学生的信息及课程分数
1.1 查询同时存在" 01 "课程和" 02 "课程的情况
1.2 查询存在" 01 "课程但可能不存在" 02 "课程的情况(不存在时显示为 null )
1.3 查询不存在" 01 "课程但存在" 02 "课程的情况
2.查询平均成绩大于等于 60 分的同学的学生编号和学生姓名和平均成绩
3.查询在 SC 表存在成绩的学生信息
4.查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩(没成绩的显示为 null )
4.1 查有成绩的学生信息
5.查询「李」姓老师的数量
6.查询学过「张三」老师授课的同学的信息
7.查询没有学全所有课程的同学的信息
8.查询至少有一门课与学号为" 01 "的同学所学相同的同学的信息
9.查询和" 01 "号的同学学习的课程 完全相同的其他同学的信息
10.查询没学过"张三"老师讲授的任一门课程的学生姓名
答案：
1.查询" 01 "课程比" 02 "课程成绩高的学生的信息及课程分数
思路:找到学生'01'和'02'课程分数(sc,2个表),要比较大小(sc),学生信息(student)
select a.*,b.score as s1,c.score as s2
from student a,sc b,sc c
where a.id=b.sid
and a.id=c.sid
and b.cid='01'
and c.cid='02'
and b.score>c.score
或者:
select a.*,b.score as s1,c.score as s2
from student a
left join (select sid,score from sc where cid='01') b
on a.id=b.sid
left join (select sid,score from sc where cid='02') c
on a.id=c.sid
where b.score>c.score;
或者:
select * from student right join
(select t1.sid, class1, class2 from
(select sid, score as class1 from sc where sc.cid = '01') t1,
(select sid, score as class2 from sc where sc.cid = '02') t2
where t1.sid= t2.sid and t1.class1 > t2.class2
) r
on student.id = r.sid
1.1查询同时存在" 01 "课程和" 02 "课程的情况
select * from
(select sid,score from sc where cid='01') a,
(select sid,score from sc where cid='02') b
where a.sid=b.sid;
1.2查询存在" 01 "课程但可能不存在" 02 "课程的情况(不存在时显示为 null )
select * from
(select sid,score from sc where cid='01') a
left join
(select sid,score from sc where cid='02') b
on a.sid=b.sid;
1.3查询不存在" 01 "课程但存在" 02 "课程的情况
select * from
(select sid,score from sc where cid='01') a
join
(select sid,score from sc where cid='02') b
on a.sid=b.sid;
2.查询平均成绩大于等于 60分的同学的学生编号和学生姓名和平均成绩
思路:编号/姓名(student),平均成绩>60分(sc)
select a.id,a.name,avg_score
from student a,
(select sid,cast(avg(score) as decimal(8,2)) as avg_score from student
group by sid
having avg(score)>=60) b
where a.id=b.sid;
3.查询在 SC 表存在成绩的学生信息
思路:成绩(sc),学生信息(student)
select distinct a.*
from student a
right join sc
on a.id=sc.sid
4.查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩(没成绩的显示为 null )
思路:编号/姓名(student),选课数,总成绩(sc)
select a.id,a.name,b.count_c,b.sum_c
from student a
left join
(select sid,count(*) as count_c,sum(score) as sum_c from sc group by sid) b
on a.id=b.sid;
5.查询「李」姓老师的数量
select count(name) from teacher
where name like '李%';
6.查询学过「张三」老师授课的同学的信息
思路:张三老师授的课(teacher,course),分数(sc),学生信息(student)
select a.*
from student a,sc,course c,teacher d
where a.id=sc.sid
and sc.cid=c.id
and c.tid=d.id
and d.name='张三'
7.查询没有学全所有课程的同学的信息
思路:全部课程数(course),每个同学学过的课程数(sc),学生信息(student)
select a.*
from student a,
(select sid,count(*) from sc
group by sid
having count(*)<(select count(*) from course)) b
where a.id=b.sid;
8.查询至少有一门课与学号为" 01 "的同学所学相同的同学的信息
思路:'01'学生学过的课程(sc),学生信息(student)+学号<>'01'
select distinct a.* from student a,sc
where a.id=sc.sid
and sc.cid in (select cid from sc where sid='01')
and sc.sid<>'01';
9.查询和" 01 "号的同学学习的课程 完全相同的其他同学的信息
思路:'01'学过的课程(sc)名称和数量,学生信息(student)
select a.* from student a,
(select sid,count(*) from sc
where cid in(select cid from sc where sid='01')
group by sid
having count(*)=(select count(*) from sc where sid='01')) b
where a.id=b.sid
and a.id<>'01';
思考:能简化吗？？？
10.查询没学过"张三"老师讲授的任一门课程的学生姓名
思路:张三教过的课程(teacher,course,sc),学生姓名(student)
select a.name from student a
where id not in
(select distinct sc.sid
from teacher b,course c,sc
where b.id=c.tid
and b.name='张三'
and c.id=sc.cid)
