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)

分享一些常见的SQL语句，包含单表查询、高级查询（连接查询、复合条件查询、嵌套查询等）。

--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
)