你必须掌握的一些常见的SQL语句，包含单表查询、高级查询（连接查询、复合条件查询、嵌套查询)

--建立学生信息表Students
create table Students
(
    SId char(5) not null primary key,
    SName nvarchar(20) unique,
    SGender char(10) default('Male'),
    SAge int,
    SSdept nvarchar(250)
    
)
--课程表
create table Course
(
    CNo Char(4) not null primary key,
    CName nvarchar(50),
    CPNo char(4),
    CCreadit smallint
    foreign key(cpno) references course(cno)
    
    
)
--学生选课表
create table StudentCourse
(
    SCId char(5) not null ,
    SCCNo char(4) not null,
    SCGrade smallint,
    primary key(Scid,sccno),
    foreign key(SCId) references Students(Sid),
    foreign key(sccno) references Course(cno)
)

--查询每个系的学生人数
select COUNT(*) as '人数',SSdept as '所在系' 
from Students group by SSdept
--查询计算机系男女生人数
select COUNT(*) as '人数',SGender 性别 
from Students where ssdept='计算机科学与技术' group by SGender
--查询每个系男女生人数
select COUNT(*) as '人数',SSdept as '系',SGender as '性别' 
from students group by ssdept,SGender
--查询男生人数超过1的系
select ssdept as '所在系',COUNT(*) as '人数'
from Students where SGender='Male' group by ssdept having COUNT(*)>2
--查询和张三同一系的学生学号、姓名
select SId as '学号' ,SName as '姓名'
from Students 
where SSdept=(select ssdept from Students where SName='张三') and SName<>'张三'
--查询比张三年纪大的学生的姓名、性别
select SName as '姓名',SGender as '性别' 
from Students where SAge>(select sage from students where sname='张三')

--查询张三的学号和其选修的课程号和成绩
select sc.SCCNo as '课程号',sc.SCGrade as '成绩'
from students as s,Course c,StudentCourse sc
where s.SId=sc.SCId and sc.SCCNo=c.CNo and s.SName='张三'

--查询张三选修高等数学上这门课的成绩

select sc.SCCNo as '课程号',c.CName as '课程名',sc.SCGrade as '成绩'
from students as s,Course c,StudentCourse sc
where s.SId=sc.SCId and sc.SCCNo=c.CNo and c.CName='高等数学上' and s.SName='张三'

--查询与张三一样大的学生姓名，性别，年龄。
select SName as '姓名',SGender as '性别',SAge as '年龄' from Students
where SAge=(select SAge from Students where SName='张三') and SName<>'张三'

--查询选修了高等数学上的学生的学号、姓名
select s.SId as '学号',s.SName as '姓名'
from students as s,Course c,StudentCourse sc
where s.SId=sc.SCId and sc.SCCNo=c.CNo and c.CName='高等数学上'

--查询张三选修的所有课程号、课程名称、成绩
select sc.SCCNo as '课程号',c.CName as '课程名',sc.SCGrade as '成绩'
from students as s,Course c,StudentCourse sc
where s.SId=sc.SCId and sc.SCCNo=c.CNo  and s.SName='张三'

--查询学习了张三选修某门课程的学生学号、姓名、年龄
select * from Students s1,StudentCourse sc1 where sc1.SCCNo in
(
    select sc.SCCNo from Students s,StudentCourse sc
    where sc.SCId=s.SId and s.SName='张三' 
)
and  sc1.SCId=s1.SId

--查询张三选修的所有课程的课程号、课程名称

select sc.SCCNo as '课程号',c.CName as '课程名' from Students s,Course c,StudentCourse sc
where s.SId=sc.SCId and sc.SCCNo=c.CNo and s.SName='张三'
--查询比张三年龄大的学生学号、姓名
select SId as '学号',SName as '姓名' from Students 
where SAge>(select SAge from Students where SName='张三')

--查询选修每门课程中成绩小于其平均成绩的学生学号
select sc1.SCId as '学生学号' from StudentCourse sc1 where SCGrade<
(
  select AVG(SCGrade) from StudentCourse sc2 where sc2.SCCNo=sc1.SCCNo
)

--查询张三选修高等数学上的课程的成绩
select * from StudentCourse  sc ,Students s,Course c
where sc.SCId=s.SId and sc.SCCNo=c.CNo and s.SName='张三' and c.CName='高等数学上'

select * from StudentCourse where SCCNo=
(
    Select Cno from Course where CName='高等数学上'
)
and
SCId=
(
    select sid from Students where  SName='张三'

)

--查询张三选修课程的平均成绩
select AVG(sc.SCGrade) as '平均成绩' from StudentCourse  sc ,Students s
where sc.SCId=s.SId and s.SName='张三'

--查询选修课程的平均成绩小于张三平均成绩的学生学号
    
 select sC.SCId from StudentCourse sc group by sc.SCId having AVG(SC.SCGrade)< 
 (
    
 select AVG(sc.SCGrade) from StudentCourse sc ,Students s where sc.SCId=s.SId and s.SName='张三'
 )

--查询课程的平均成绩低于张三平均成绩的课程号

 select sC.SCCNo from StudentCourse sc group by sc.SCCNo having AVG(SC.SCGrade)< 
 (
    
         select AVG(sc.SCGrade) from StudentCourse sc ,Students s where sc.SCId=s.SId and s.SName='张三'
 )
 
--查询选修课程成绩大于等于该课程平均成绩的学生学号
 
select SCId from StudentCourse sc1 where  sc1.SCGrade >=
(
     select avg( sc2.SCGrade ) from StudentCourse sc2  where sc2.SCCNo=sc1.SCCNo
)