sql 综合练习

一、准备数据

1、创建学生表

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

1.1向学生表中插入数据

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、创建课程表

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

2.1向课程表中插入数据

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、 创建教师表

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

3.1向教师表中插入数据

insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');

4、 创建成绩表

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

4.1向成绩表中插入数据

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);

二、练习

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 '01课程成绩'
,c.score '02课程成绩'
from student a 
join sc b on a.`SID`=b.`SID`
join sc c on b.`SID`=c.`SID`
where b.`CID`='01' and c.`CID`=02 and b.score>c.score

结果：

-- 1.2、查询同时存在"01"课程和"02"课程的情况和存在"01"课程但可能不存在"02"课程的情况(不存在时显示为null)(以下存在相同内容时不再解释)

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 > isnull(c.score)

结果：

 2、查询"01"课程比"02"课程成绩低的学生的信息及课程分数

2.1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 
join sc b on a.`SID`=b.`SID`
join sc c on a.`SID`=c.`SID`
where b.`CID`='01' and c.`CID`='02' and b.score<c.score 

结果：

2.2、查询同时存在"01"课程和"02"课程的情况和不存在"01"课程但存在"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 
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 isnull(b.score)<c.score 

结果：

 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

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

结果：

 ①cast函数运用

CAST函数用于将某种数据类型的表达式显式转换为另一种数据类型。CAST()函数的参数是一个表达式，它包括用AS关键字分隔的源值和目标数据类型。

语法：CAST (expression AS data_type)

• expression：任何有效的SQServer表达式。
• AS：用于分隔两个参数，在AS之前的是要处理的数据，在AS之后是要转换的数据类型。
• data_type：目标系统所提供的数据类型，包括bigint和sql_variant，不能使用用户定义的数据类型。

data_type类型

• 二进制，同带binary前缀的效果 : BINARY    
• 字符型，可带参数 : CHAR()     
• 日期 : DATE     
• 时间: TIME     
• 日期时间型 : DATETIME     
• 浮点数 : DECIMAL      
• 整数 : SIGNED     
• 无符号整数 : UNSIGNED

SELECT  CAST(NOW() AS DATE)

结果：

②decimal运用

decimal(18,3)中的“2”表示小数部分的位数，如果插入的值未指定小数部分或者小数部分不足两位则会自动补3位小数，若插入的值小数部分超过了3为则会发生截断，截取前3位小数。

“18”指的是整数部分加小数部分的总长度，也即插入的数字整数部分不能超过“18-2”位，否则不能成功插入，会报超出范围的错误。

select cast(1234.76543 as decimal(18,3)) 

结果：

4、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩--4.1、查询在sc表存在成绩的学生信息的SQL语句。

select b.*    
        ,cast(avg(a.score) as decimal(18,2)) avg_score
from sc a
left join student b on a.`SID`=b.`SID`
group by a.`SID`
having avg_score<60
order by avg_score desc

结果：

 4.2查询在sc表中不存在成绩的学生信息的SQL语句。

select a.SID 
        , a.Sname 
        , cast(avg(b.score) as decimal(18,2)) avg_score
from Student a left join sc b on a.SID = b.SID
group by a.SID , a.Sname
having ifnull(cast(avg(b.score) as decimal(18,2)),0) < 60
order by a.SID

结果：

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`
        ,a.`Sname`
        ,count(distinct b.`CID`) 选课总数
        ,sum(b.score) 总成绩
from student a
join sc b on a.`SID`=b.`SID`
group by a.`SID`

 5.2、查询所有(包括有成绩和无成绩)的SQL。

select a.`SID`
        ,a.`Sname`
        ,count(b.`CID`) 选课总数
        ,sum(b.score) 总成绩
from student a
left join sc b on a.`SID`=b.`SID`
group by a.`Sname`
order by a.`SID` desc

 6、查询"李"姓老师的数量

方法1

select count(a.`Tname`) 李姓老师的数量
from teacher a
where a.`Tname` like "李%"

方法2

select count(Tname) 李姓老师的数量 
from Teacher 
where left(Tname,1) = '李'

 7、查询学过"张三"老师授课的同学的信息

select distinct d.*
from teacher a
join course b on a.`TID`=b.`TID`
join sc c on b.`CID`=c.`CID`
join student d on c.`SID` = d.`SID`
where a.`Tname` = '张三'

 8、查询没学过"张三"老师授课的同学的信息

select *
from student m
where m.`SID` not in
(select c.`SID`
from teacher a
join course b on a.`TID`=b.`TID`
join sc c on b.`CID`=c.`CID`
where a.`Tname`='张三')

 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

方法1

select a.*
from student a
join sc b on a.`SID`=b.`SID` and b.`CID`='01'
join sc c on b.`SID`=c.`SID` and c.`CID`='02'

方法2

select a.*
from student a
join sc  on a.`SID`=sc.`SID` and sc.`CID`='01'
where exists (select * from sc c 
where c.`SID`=sc.`SID` and c.`CID`='02')

方法3

select m.* from Student m where SID in
(
select SID from
(
select * from SC where CID = '01'
union all
select * from SC where CID = '02'
) t group by SID having count(`SID`) = 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

方法1

select a.*
from student a 
join sc b on a.`SID`=b.`SID` and b.`CID`='01'
where not exists(select * from sc c where c.`SID`=b.`SID` 
and c.`CID`='02')
order by a.`SID`

方法2

select a.*
from student a 
join sc b on a.`SID`=b.`SID` and b.`CID`='01'
where a.`SID`not in 
(select c.`SID` from sc c where b.`SID`=c.`SID` and c.`CID`='02')

11、查询没有学全所有课程的同学的信息

①

select a.*
from student a
join sc b on a.`SID`=b.`SID`
group by b.`SID`
having count(b.`CID`) !=3

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

select distinct a.*
from student a
join sc b on a.`SID`=b.`SID`
where b.`CID` in (select c.`CID` from sc c where c.`SID`='01' )
and a.`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

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'))

 14、查询没学过"张三"老师讲授的任一门课程的学生姓名

select *
from student m
where m.`Sname`not in(
select distinct a.`Sname`
from student a
join sc b on a.`SID`=b.`SID`
join course c on c.`CID` = b.`CID`
join teacher d on d.`TID`=c.`TID`
where d.`Tname`='张三')

 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

select a.`SID`
        ,a.`Sname`
        ,cast(avg(b.score) as decimal(18,2)) as 平均成绩
from student a 
join sc b on a.`SID`=b.`SID`
where a.`SID` in (select b.`SID`
from sc b  where b.score<60)
group by b.`SID` having count(b.`SID`)>1

16、检索"01"课程分数小于60，按分数降序排列的学生信息

select a.*
        ,b.`CID`
        ,b.score
from student a
join sc b on a.`SID`=b.`SID`
where b.`CID`='01' and b.score<60
order by b.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--17.1 SQL 2000 静态

select a.`SID` 学号
        ,a.`Sname` 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
        ,cast(avg(b.score) as decimal(18,2)) 平均分
from student a
left join sc b on a.`SID`=b.`SID`
join course c on b.`CID`=c.`CID`
group by a.`SID`
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

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 sc.`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

--19、按各科成绩进行排序，并显示排名--19.1 sql 2000用子查询完成

Score重复时保留名次空缺

select t.* , 
(select count(1) from SC where CID = t.CID and score > t.score) + 1 r 
from sc t 
order by t.`CID`,r 

Score重复时合并名次

select t.* ,
(select count(distinct score) from SC where CID = t.CID and score >= t.score) r 
from sc t order by t.`CID`,r

 20、查询学生的总成绩并进行排名--20.1 查询学生的总成绩

select b.*
        ,ifnull(sum(a.score),0) 总成绩
from student b
left join sc a on a.`SID`=b.`SID`
group by b.`Sname`
order by 总成绩 desc

 20.2 查询学生的总成绩并进行排名，sql 2000用子查询完成，分总分重复时保留名次空缺和不保留名次空缺两种。

①

select t1.* ,(select count(1) from
(
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) t2 
where 总成绩 > t1.总成绩) + 1 px from
(
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
) t1
order by px

 ②

select t1.* ,(select count(distinct 总成绩) from
(
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
) t2 where 总成绩 >= t1.总成绩) px from
(
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
) t1
order by px

 21、查询不同老师所教不同课程平均分从高到低显示

select a.`Tname`
        ,b.`CID`
        ,round(avg(c.score),2) 平均分
from teacher a 
join course b on a.`TID`=b.`TID`
join sc c on b.`CID`=c.`CID`
group by a.`TID`
order by 平均分 desc

22、查询所有课程的成绩第2名到第3名的学生信息及该课程成绩--22.1 sql 2000用子查询完成

①Score重复时保留名次空缺

select * from (select t.* ,
(select count(1) from SC where CID = t.CID and score > t.score) + 1 px 
from sc t) m join student a on m.SID=a.`SID`
where px between 2 and 3 order by m.cid , m.px

 

 ②Score重复时合并名次

select * from (select t.* ,
(select count(distinct score) from SC where CID = t.CID and score >= t.score) px
from sc t) m join student a on m.SID=a.`SID`
where px between 2 and 3 order by m.cid , m.px

 23、统计各科成绩各分数段人数：课程编号,课程名称, 100-85 , 85-70 , 70-60 , 0-60 及所占百分比 

23.1 统计各科成绩各分数段人数：课程编号,课程名称, 100-85 , 85-70 , 70-60 , 0-60

①横向显示

select Course.CID 课程编号 , Cname as 课程名称 ,
sum(case when score >= 85 then 1 else 0 end) '85-100' ,
sum(case when score >= 70 and score < 85 then 1 else 0 end) '70-85' ,
sum(case when score >= 60 and score < 70 then 1 else 0 end) '60-70' ,
sum(case when score < 60 then 1 else 0 end) '0-60'
from sc , Course
where SC.CID = Course.CID
group by Course.CID , Course.Cname
order by Course.CID

 ②纵向显示1(显示存在的分数段)

select m.CID 课程编号 , m.Cname 课程名称 ,(
case when n.score >= 85 then '85-100'
when n.score >= 70 and n.score < 85 then '70-85'
when n.score >= 60 and n.score < 70 then '60-70'
else '0-60'
end) 分数段 ,
count(1) 数量
from Course m , sc n
where m.CID = n.CID
group by m.CID , m.Cname , (
case when n.score >= 85 then '85-100'
when n.score >= 70 and n.score < 85 then '70-85'
when n.score >= 60 and n.score < 70 then '60-70'
else '0-60'
end)
order by m.CID , m.Cname , 分数段

23.2 统计各科成绩各分数段人数：课程编号,课程名称, 100-85 , 85-70 , 70-60 , <60 及所占百分比

①横向显示

select m.CID 课程编号, m.Cname 课程名称,
(select count(1) from SC where CID = m.CID and score < 60) '0-60' ,
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)) 百分比 ,
(select count(1) from SC where CID = m.CID and score >= 60 and score < 70) '60-70' ,
cast((select count(1) from SC where CID = m.CID and score >= 60 and score < 70)*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 百分比 ,
(select count(1) from SC where CID = m.CID and score >= 70 and score < 85) '70-85' ,
cast((select count(1) from SC where CID = m.CID and score >= 70 and score < 85)*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 百分比 ,
(select count(1) from SC where CID = m.CID and score >= 85) '85-100' ,
cast((select count(1) from SC where CID = m.CID and score >= 85)*100.0 / (select count(1) from SC where CID = m.CID) as decimal(18,2)) 百分比
from Course m
order by m.CID

②纵向显示1(显示存在的分数段)

select m.CID 课程编号 , m.Cname 课程名称 , (
case when n.score >= 85 then '85-100'
when n.score >= 70 and n.score < 85 then '70-85'
when n.score >= 60 and n.score < 70 then '60-70'
else '0-60'
end)分数段 ,
count(1) 数量 ,
cast(count(1) * 100.0 / (select count(1) from sc where CID = m.CID) as decimal(18,2)) 百分比
from Course m , sc n
where m.CID = n.CID
group by m.CID , m.Cname , (
case when n.score >= 85 then '85-100'
when n.score >= 70 and n.score < 85 then '70-85'
when n.score >= 60 and n.score < 70 then '60-70'
else '0-60'
end)
order by m.CID , m.Cname , 分数段

24、查询学生平均成绩及其名次--24.1 查询学生的平均成绩并进行排名，sql 2000用子查询完成，分平均成绩重复时保留名次空缺和不保留名次空缺两种。

select f1.*
        ,(select count(1) from (
select a.*
        ,ifnull(cast(avg(score) as decimal(18,2)),0)平均分
from student a left join sc b on a.`SID`=b.`SID`
group by a.`SID`
) f where 平均分>=f1.平均分) 排名
from (select a.*
        ,ifnull(cast(avg(score) as decimal(18,2)),0) 平均分
from student a left join sc b on a.`SID`=b.`SID`
group by a.`SID`) f1
order by 平均分 desc

 25、查询各科成绩前三名的记录，分数重复时不保留名次空缺，合并名次

select * from 
(select t.* ,
(select count(distinct score) from SC where CID = t.CID and score >= t.score) px 
from sc t) m 
where px between 1 and 3 order by m.Cid , m.px

 26、查询每门课程被选修的学生数

select a.`CID`,count(1) 学生数
from sc a 
group by a.`CID`

 27、查询出只有两门课程的全部学生的学号和姓名

select a.SID , a.Sname
from Student a , SC b 
where a.SID = b.SID
group by a.SID , a.Sname
having count(b.CID) = 2
order by a.SID

 28、查询男生、女生人数

①

select sum(case when Ssex = N'男' then 1 else 0 end) 男生人数 ,sum(case when Ssex = N'女' then 1 else 0 end) 女生人数 from student

 ②

select case when Ssex = N'男' then N'男生人数' else N'女生人数' end 男女情况 , count(1) 人数 from student group by case when Ssex = N'男' then N'男生人数' else N'女生人数' end

 29、查询名字中含有"风"字的学生信息

select * from student where sname like '%风%'

 30、查询同名同性学生名单，并统计同名人数

select Sname 学生姓名 , count(*) 人数 from Student group by Sname having count(*) > 1

31、查询1990年出生的学生名单(注：Student表中Sage列的类型是datetime)

select * from Student where year(sage) = 1990

32、查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列

select a.`CID`,b.`Cname`,cast(avg(a.score) as decimal(18,1)) 平均分 
from sc a,course b
where a.`CID`=b.`CID`
group by a.`CID`
order by 平均分 desc,a.`CID`

 33、查询平均成绩大于等于85的所有学生的学号、姓名和平均成绩

select a.`SID`,a.`Sname`,cast(avg(b.score) as decimal(18,1)) 平均成绩
from student a,sc b
where a.`SID`=b.`SID`
group by a.`Sname`
having 平均成绩>=85

 34、查询课程名称为"数学"，且分数低于60的学生姓名和分数

select b.`Sname`,c.score
from course a,student b,sc c
where a.`CID`=c.`CID` and c.`SID`=b.`SID` and a.`Cname`='数学' 
having c.score<60

 35、查询所有学生的课程及分数情况

select Student.* , Course.Cname , SC.CID , SC.score
from Student, SC , Course
where Student.SID = SC.SID and SC.CID = Course.CID
order by Student.SID , SC.CID

 36、查询任何一门课程成绩在70分以上的姓名、课程名称和分数

select a.`Sname`,c.`Cname`,b.score
from student a,sc b,course c
where a.`SID`=b.`SID` and b.`CID`=c.`CID` and b.score>=70

 37、查询不及格的课程

select c.*,a.`CID`,b.score
from course a,sc b,student c
where a.`CID`=b.`CID`and b.`SID`=c.`SID`and b.score<60

 38、查询课程编号为01且课程成绩在80分以上的学生的学号和姓名

select b.`SID`,b.`Sname`,a.*
from sc a,student b
where a.`SID`=b.`SID`and a.score>=80 and a.`CID`='01'

 39、求每门课程的学生人数

select a.`CID`,count(*) 学生人数
from sc a,student b
where a.`SID`=b.`SID`
group by a.`CID`

 40、查询选修"张三"老师所授课程的学生中，成绩最高的学生信息及其成绩--40.1 当最高分只有一个时

select b.*,a.score
from sc a,student b,course c,teacher d
where a.`SID`=b.`SID` and a.`CID`=c.`CID`and d.`TID`=c.`TID`and d.`Tname`like '张三'
order by a.score desc 
limit 1)

 40.2 当最高分出现多个时

select b.*,a.score
from sc a,student b,course c,teacher d
where a.`SID`=b.`SID` and a.`CID`=c.`CID`and d.`TID`=c.`TID`and d.`Tname`like '张三'
and a.score=(select a.score
from sc a,student b,course c,teacher d
where a.`SID`=b.`SID` and a.`CID`=c.`CID`and d.`TID`=c.`TID`and d.`Tname`like '张三'
order by a.score desc 
limit 1)

41、查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩

①

select m.* from SC m 
,(select CID , score from SC group by CID , score having count(1) > 1) n
where m.CID= n.CID and m.score = n.score 
order by m.CID , m.score , m.SID

②

select m.* from SC m where exists (select 1 from (select CID , score from SC group by CID , score having count(1) > 1) n
where m.CID= n.CID and m.score = n.score) 
order by m.CID , m.score , m.SID

 42、查询每门课成绩最好的前两名

SELECT s1.* FROM sc s1 WHERE
(
SELECT COUNT(1) FROM sc s2 WHERE
s1.`CID`=s2.`CID` AND s2.score>=s1.score
)<=2
ORDER BY s1.`CID`,s1.score DESC;

43、统计每门课程的学生选修人数（超过5人的课程才统计）。要求输出课程号和选修人数，查询结果按人数降序排列，若人数相同，按课程号升序排列

select a.`CID`,b.`Cname`,count(a.`SID`) 选修人数
from sc a,course b
where a.`CID`=b.`CID`
group by a.`CID`
having 选修人数>5
order by 选修人数 desc,a.`CID`

44、检索至少选修两门课程的学生学号

select a.*,count(*) 课程数
from student a,sc b
where a.`SID`=b.`SID`
group by a.`SID`
having 课程数>=2

 45、查询选修了全部课程的学生信息

①根据数量来完成

select student.* from student where SID in
(select SID from sc group by SID 
having count(1) = (select count(1) from course))

②使用双重否定来完成

select t.* from student t 
where t.SID not in
(
select distinct m.SID from
(
select SID , CID from student , course
) m 
where not exists 
(select 1 from sc n where n.SID = m.SID and n.CID = m.CID)
)

③使用双重否定来完成

select t.* from student t 
where not exists(select 1 from
(
select distinct m.SID from
(
select SID , CID from student , course
) m where not exists 
(select 1 from sc n where n.SID = m.SID and n.CID = m.CID)
) k where k.SID = t.SID
)

 46、查询各学生的年龄--46.1 只按照年份来算

①

select student.*,date_format(now(),'%Y')-date_format(student.Sage,'%Y') as age from student;

②

select a.*,year(now())-year(a.`Sage`) 年龄
from student a

46.2 按照出生日期来算，当前月日 < 出生年月的月日则，年龄减一

select a.*, 
year(a.`Sage`),
(case when dayofyear(now())<dayofyear(a.`Sage`) 
 then year(now())-year(a.`Sage`)-1
 else year(now())-year(a.`Sage`) 
 end)'年龄'
from student a

47、查询本周过生日的学生

select a.*
from student a
where abs(dayofyear(now())-dayofyear(a.`Sage`))<7 and dayofyear(now())-dayofyear(a.`Sage`)<0

48、查询下周过生日的学生

select a.*
from student a
where abs(dayofyear(now())-dayofyear(a.`Sage`))<=14 
and 7<abs(dayofyear(now())-dayofyear(a.`Sage`)) 
and dayofyear(now())-dayofyear(a.`Sage`)<0

49、查询本月过生日的学生

select a.*,month(a.Sage) 月份
from student a
where month(a.Sage)=month(now())

50、查询下月过生日的学生

select a.*,month(a.Sage) 月份
from student a
where month(a.Sage)=month(now())+1