你必須掌握的一些常見的SQL語句，包含單表查詢、高級查詢（連接查詢、復合條件查詢、嵌套查詢)

分享一些常見的SQL語句，包含單表查詢、高級查詢（連接查詢、復合條件查詢、嵌套查詢等）。

--建立學生信息表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)
    
)
--課程表
create table Course
(
    CNo Char(4) not null primary key,
    CName nvarchar(50),
    CPNo char(4),
    CCreadit smallint
    foreign key(cpno) references course(cno)
    
    
)
--學生選課表
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)
)

--查詢每個系的學生人數
select COUNT(*) as '人數',SSdept as '所在系' 
from Students group by SSdept
--查詢計算機系男女生人數
select COUNT(*) as '人數',SGender 性別 
from Students where ssdept='計算機科學與技術' group by SGender
--查詢每個系男女生人數
select COUNT(*) as '人數',SSdept as '系',SGender as '性別' 
from students group by ssdept,SGender
--查詢男生人數超過1的系
select ssdept as '所在系',COUNT(*) as '人數'
from Students where SGender='Male' group by ssdept having COUNT(*)>2
--查詢和張三同一系的學生學號、姓名
select SId as '學號' ,SName as '姓名'
from Students 
where SSdept=(select ssdept from Students where SName='張三') and SName<>'張三'
--查詢比張三年紀大的學生的姓名、性別
select SName as '姓名',SGender as '性別' 
from Students where SAge>(select sage from students where sname='張三')

--查詢張三的學號和其選修的課程號和成績
select sc.SCCNo as '課程號',sc.SCGrade as '成績'
from students as s,Course c,StudentCourse sc
where s.SId=sc.SCId and sc.SCCNo=c.CNo and s.SName='張三'

--查詢張三選修高等數學上這門課的成績

select sc.SCCNo as '課程號',c.CName as '課程名',sc.SCGrade as '成績'
from students as s,Course c,StudentCourse sc
where s.SId=sc.SCId and sc.SCCNo=c.CNo and c.CName='高等數學上' and s.SName='張三'

--查詢與張三一樣大的學生姓名，性別，年齡。
select SName as '姓名',SGender as '性別',SAge as '年齡' from Students
where SAge=(select SAge from Students where SName='張三') and SName<>'張三'

--查詢選修了高等數學上的學生的學號、姓名
select s.SId as '學號',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='高等數學上'

--查詢張三選修的所有課程號、課程名稱、成績
select sc.SCCNo as '課程號',c.CName as '課程名',sc.SCGrade as '成績'
from students as s,Course c,StudentCourse sc
where s.SId=sc.SCId and sc.SCCNo=c.CNo  and s.SName='張三'

--查詢學習了張三選修某門課程的學生學號、姓名、年齡
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='張三' 
)
and  sc1.SCId=s1.SId

--查詢張三選修的所有課程的課程號、課程名稱

select sc.SCCNo as '課程號',c.CName as '課程名' from Students s,Course c,StudentCourse sc
where s.SId=sc.SCId and sc.SCCNo=c.CNo and s.SName='張三'
--查詢比張三年齡大的學生學號、姓名
select SId as '學號',SName as '姓名' from Students 
where SAge>(select SAge from Students where SName='張三')

--查詢選修每門課程中成績小於其平均成績的學生學號
select sc1.SCId as '學生學號' from StudentCourse sc1 where SCGrade<
(
  select AVG(SCGrade) from StudentCourse sc2 where sc2.SCCNo=sc1.SCCNo
)

--查詢張三選修高等數學上的課程的成績
select * from StudentCourse  sc ,Students s,Course c
where sc.SCId=s.SId and sc.SCCNo=c.CNo and s.SName='張三' and c.CName='高等數學上'

select * from StudentCourse where SCCNo=
(
    Select Cno from Course where CName='高等數學上'
)
and
SCId=
(
    select sid from Students where  SName='張三'

)

--查詢張三選修課程的平均成績
select AVG(sc.SCGrade) as '平均成績' from StudentCourse  sc ,Students s
where sc.SCId=s.SId and s.SName='張三'

--查詢選修課程的平均成績小於張三平均成績的學生學號
    
 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='張三'
 )

--查詢課程的平均成績低於張三平均成績的課程號

 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='張三'
 )
 
--查詢選修課程成績大於等於該課程平均成績的學生學號
 
select SCId from StudentCourse sc1 where  sc1.SCGrade >=
(
     select avg( sc2.SCGrade ) from StudentCourse sc2  where sc2.SCCNo=sc1.SCCNo
)