SQL回顾1

1.学生表
Student(SID,Sname,Sage,Ssex) --SID 学生编号,Sname 学生姓名,Sage 出生年月,Ssex 学生性别

2.课程表

Course(CID,Cname,TID) --CID --课程编号,Cname 课程名称,TID 教师编号

3.教师表

Teacher(TID,Tname) --TID 教师编号,Tname 教师姓名

4.成绩表

SC(SID,CID,score) --SID 学生编号,CID 课程编号,score 分数

添加测试数据
1.学生表

create table Student(SID varchar(10),Sname nvarchar(10),Sage datetime,Ssex varchar(10));

insert into Student values('01' , '赵雷' , '1990-01-01' , '男');

insert into Student values('02' , '钱电' , '1990-12-21' , '男');

insert into Student values('03' , '孙风' , '1990-05-20' , '男');

insert into Student values('04' , '李云' , '1990-08-06' , '男');

insert into Student values('05' , '周梅' , '1991-12-01' , '女');

insert into Student values('06' , '吴兰' , '1992-03-01' , '女');

insert into Student values('07' , '郑竹' , '1989-07-01' , '女');

insert into Student values('08' , '王菊' , '1990-01-20' , '女');

2.课程表

create table Course(CID varchar(10),Cname nvarchar(10),TID varchar(10));

insert into Course values('01' , '语文' , '02');

insert into Course values('02' , '数学' , '01');

insert into Course values('03' , '英语' , '03');

3.教师表

create table Teacher(TID varchar(10),Tname nvarchar(10));

insert into Teacher values('01' , '张三');

insert into Teacher values('02' , '李四');

insert into Teacher values('03' , '王五');

4.成绩表

create table SC(SID varchar(10),CID varchar(10),score decimal(18,1));

insert into SC values('01' , '01' , 80);

insert into SC values('01' , '02' , 90);

insert into SC values('01' , '03' , 99);

insert into SC values('02' , '01' , 70);

insert into SC values('02' , '02' , 60);

insert into SC values('02' , '03' , 80);

insert into SC values('03' , '01' , 80);

insert into SC values('03' , '02' , 80);

insert into SC values('03' , '03' , 80);

insert into SC values('04' , '01' , 50);

insert into SC values('04' , '02' , 30);

insert into SC values('04' , '03' , 20);

insert into SC values('05' , '01' , 76);

insert into SC values('05' , '02' , 87);

insert into SC values('06' , '01' , 31);

insert into SC values('06' , '03' , 34);

insert into SC values('07' , '02' , 89);

insert into SC values('07' , '03' , 98);

--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

select a.* , b.score 课程01的分数,c.score 课程02的分数 from Student a , SC b , SC c

where a.SID = b.SID and a.SID = c.SID and b.CID = '01' and c.CID = '02' and b.score > c.score;

--1.2、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 a.* , b.score 课程01的分数,c.score 课程02的分数 from Student a

left join SC b on a.SID = b.SID and b.CID = '01'

left join SC c on a.SID = c.SID and c.CID = '02'

where b.score > ifnull(c.score,0);
此语句中student表为左表
ifnull(a,b):若a不为null则返回a,否则返回b

--3、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
两者道理相同
我的方法：
select a.sid, a.sname, avg(b.score)
from student a,sc b
where a.sid = b.sid group by a.sid having avg(b.score) >= 60;
答案的方法：
select a.SID , a.Sname , cast(avg(b.score) as decimal(18,2)) avg_score

from Student a , sc b

where a.SID = b.SID

group by a.SID , a.Sname

having cast(avg(b.score) as decimal(18,2)) >= 60

order by a.SID

Cast(字段名 as 转换的类型 )
cast(a as b):将a字段的值转换为b类型

--4.2、查询在sc表中不存在成绩的学生信息的SQL语句。(此处的不存在成绩指的是选了课之后没有成绩的同学，而不是为选课的同学，容易误写为
select *
from student
where sid
not in (select distinct sid from sc);)
实际是查询成绩为空的同学如下：
select a.* from student a where a.sid in (select sc.sid from sc where ifnull(sc.score,0) = 0);

--5、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--5.1、查询所有有成绩的SQL。

select a.SID 学生编号 , a.Sname 学生姓名 , count(b.CID) 选课总数, sum(score) 所有课程的总成绩

from Student a , SC b

where a.SID = b.SID

group by a.SID,a.Sname

order by a.SID

--5.2、查询所有(包括有成绩和无成绩)的SQL。

select a.SID 学生编号 , a.Sname 学生姓名 , count(b.CID) 选课总数, sum(score) 所有课程的总成绩

from Student a left join SC b

on a.SID = b.SID

group by a.SID,a.Sname

order by a.SID

--7、查询学过"张三"老师授课的同学的信息（此语句中使用distinct是因为张三老师教过不只一门课）

select distinct Student.* from Student , SC , Course , Teacher

where Student.SID = SC.SID and SC.CID = Course.CID and Course.TID = Teacher.TID and Teacher.Tname = '张三'

order by Student.SID

--8、查询没学过"张三"老师授课的同学的信息

select m.* from Student m where SID not in (select distinct SC.SID from SC , Course , Teacher where SC.CID = Course.CID and Course.TID = Teacher.TID and Teacher.Tname = '张三') order by m.SID

--9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息
我的方法：
select *
from student
where sid in (select sid from sc where cid = '01')
and sid in (select sid from sc where cid = '02');

--方法1

select Student.* from Student , SC where Student.SID = SC.SID and SC.CID = '01' and exists (Select 1 from SC SC_2 where SC_2.SID = SC.SID and SC_2.CID = '02') order by Student.SID

--方法2

select Student.* from Student , SC where Student.SID = SC.SID and SC.CID = '02' and exists (Select 1 from SC SC_2 where SC_2.SID = SC.SID and SC_2.CID = '01') order by Student.SID

--方法3

select m.* from Student m where SID in

(

select SID from

(

select distinct SID from SC where CID = '01'

union all

select distinct SID from SC where CID = '02'

) t group by SID having count(1) = 2

)

order by m.SID

--10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息
我的方法1

select student.*
from student where student.sid in(select sid from sc where cid = '01' and sid not in (select sid from sc where cid = '02'));
我的方法2
select *
from student
where sid in (select sid from sc where cid = '01')
and sid not in (select sid from sc where cid = '02');

--方法1

select Student.* from Student , SC where Student.SID = SC.SID and SC.CID = '01' and not exists (Select 1 from SC SC_2 where SC_2.SID = SC.SID and SC_2.CID = '02') order by Student.SID

--方法2

select Student.* from Student , SC where Student.SID = SC.SID and SC.CID = '01' and Student.SID not in (Select SC_2.SID from SC SC_2 where SC_2.SID = SC.SID and SC_2.CID = '02') order by Student.SID

--11、查询没有学全所有课程的同学的信息
我的方法1:
select
a.*
from student a,sc b
where a.sid = b.sid
group by a.sid having count(a.sid) < (select count(*) from course);
我的方法2：
select *
from student
where sid in (select sid from sc group by sid having count(*) < (select count(*) from course));
--11.1、

select Student.*

from Student , SC

where Student.SID = SC.SID

group by Student.SID , Student.Sname , Student.Sage , Student.Ssex having count(CID) < (select count(CID) from Course)

--11.2

select Student.*

from Student left join SC

on Student.SID = SC.SID

group by Student.SID , Student.Sname , Student.Sage , Student.Ssex having count(CID) < (select count(CID) from Course)

--12、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
我的方法1:
select distinct student.*
from student , sc
where student.sid = sc.sid
and student.sid != '01'
and sc.cid
in (select cid from sc where sid = '01');

select distinct Student.* from Student , SC where Student.SID = SC.SID and SC.CID in (select CID from SC where SID = '01') and Student.SID <> '01'

--13、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（答案不对）
select Student.* from Student where SID in

(select distinct SC.SID from SC where SID <> '01' and SC.CID in (select distinct CID from SC where SID = '01')

group by SC.SID having count(1) = (select count(1) from SC where SID='01'))
--13、1 查询和"01"号的同学学习的课程数相同的其他同学的信息

select student.* from student where student.sid in(
select a.sid
from sc a where a.sid != '01' group by a.sid having count(a.sid) = (select count(*) from sc where sid= '01'));

--14、查询没学过"张三"老师讲授的任一门课程的学生姓名（注意此处应该使用distinct,因为张三老师可能教不只一门课）
方法1：
select student.sname
from student
where sid
in (select sid from sc
where cid
not in (select course.cid from course,teacher where course.tid = teacher.tid and teacher.tname = '张三'));

方法2：select student.* from student where student.SID not in

(select distinct sc.SID from sc , course , teacher where sc.CID = course.CID and course.TID = teacher.TID and teacher.tname = '张三')

order by student.SID

--15、查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩(易错题)
注意此处要使用最后面的group by 否则显示的是所有学生总成绩的平均值

select student.sid,student.sname,avg(sc.score)
from student,sc
where student.sid = sc.sid
and student.sid in (select distinct sid from sc where score < 60 group by sid having count(*) >= 2) group by student.sid;

select student.SID , student.sname , cast(avg(score) as decimal(18,2)) avg_score from student , sc

where student.SID = SC.SID and student.SID in (select SID from SC where score < 60 group by SID having count(1) >= 2)

group by student.SID , student.sname

--16、检索"01"课程分数小于60，按分数降序排列的学生信息

select student.* , sc.CID , sc.score from student , sc

where student.SID = SC.SID and sc.score < 60 and sc.CID = '01'

order by sc.score desc

--17、按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩

注意：
错误认识：
以前认为要把和两张表都有关联的中间表放在关联语句的中间（否则返回结果不一致），如下语句：先写的是student表，然后连接sc表，最后是course表。sc表在中间
实际原因是：不是一定要把与其他表关联的表放在中间，而是左连接会显示左表的全部内容，谁作为左表才是显示结果的关键，因此会出现左表不同显示结果不同

select a.SID 学生编号 , a.Sname 学生姓名 ,

max(case c.Cname when '语文' then b.score else null end) 语文 ,

max(case c.Cname when '数学' then b.score else null end) 数学 ,

max(case c.Cname when '英语' then b.score else null end) 英语 ,
max(case c.cname when '政治' then b.score else null end) 政治,

cast(avg(b.score) as decimal(18,2)) 平均分

from Student a

left join SC b on a.SID = b.SID

left join Course c on b.CID = c.CID

group by a.SID , a.Sname

order by 平均分 desc;

--18、查询各科成绩最高分、最低分和平均分：以如下形式显示：课程ID，课程name，最高分，最低分，平均分，及格率，中等率，优良率，优秀率

--及格为>=60，中等为：70-80，优良为：80-90，优秀为：>=90

注意此语句中虽然每个课程的总人数（即(select count(1) from SC where CID = m.CID) as decimal(18,2)语句）用到过多次，但是不能给其在第一次中进行命名，用到下一次中，会产生语法错误。
试验如下（cast((select count(1) from SC where CID = m.CID and score >= 60)*100.0 / ((select count(1) from SC where CID = m.CID) as decimal(18,2) lv)) 及格率 ,
cast((select count(1) from SC where CID = m.CID and score >= 70 and score < 80 )*100.0 / lv) 中等率 ,），报语法错误。

--方法1

select m.CID 课程编号 , m.Cname 课程名称 ,

max(n.score) 最高分 ,

min(n.score) 最低分 ,

cast(avg(n.score) as decimal(18,2)) 平均分 ,

cast((select count(1) from SC where CID = m.CID and score >= 60)*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 及格率 ,

cast((select count(1) from SC where CID = m.CID and score >= 70 and score < 80 )*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 中等率 ,

cast((select count(1) from SC where CID = m.CID and score >= 80 and score < 90 )*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 优良率 ,

cast((select count(1) from SC where CID = m.CID and score >= 90)*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 优秀率

from Course m , SC n

where m.CID = n.CID

group by m.CID , m.Cname

order by m.CID;

--方法2

select m.CID 课程编号 , m.Cname 课程名称 ,

(select max(score) from SC where CID = m.CID) 最高分 ,

(select min(score) from SC where CID = m.CID) 最低分 ,

(select cast(avg(score) as decimal(18,2)) from SC where CID = m.CID) 平均分 ,

cast((select count(1) from SC where CID = m.CID and score >= 60)*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 及格率,

cast((select count(1) from SC where CID = m.CID and score >= 70 and score < 80 )*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 中等率 ,

cast((select count(1) from SC where CID = m.CID and score >= 80 and score < 90 )*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 优良率 ,

cast((select count(1) from SC where CID = m.CID and score >= 90)*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 优秀率

from Course m

order by m.CID;

--20、查询学生的总成绩并进行排名

select m.SID 学生编号 ,

m.Sname 学生姓名 ,

ifnull(sum(score),0) 总成绩

from Student m left join SC n on m.SID = n.SID

group by m.SID , m.Sname

order by 总成绩 desc;