--建立学生信息表Studentscreate 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 scwhere 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 scwhere 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 Studentswhere 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 scwhere 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 scwhere 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 scwhere 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 cwhere 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')andSCId=(    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 swhere 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)