【SQL】常用数据库操作及面试问题总结 学生老师课程学分

--    Student(SID,Sname,Sage,Ssex)    学生表
--    Course(CID,Cname,TID)            课程表
--    SC(SID,CID,score)                成绩表
--    Teacher(TID,Tname)                教师表

USE [TestDB]
GO
/****** Object:  Table [dbo].[Teacher]    Script Date: 01/14/2013 22:30:24 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[Teacher](
    [TID] [varchar](10) NOT NULL,
    [Tname] [varchar](10) NULL,
 CONSTRAINT [PK_Teacher] PRIMARY KEY CLUSTERED 
(
    [TID] ASC
) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
INSERT [dbo].[Teacher] ([TID], [Tname]) VALUES (CONVERT(TEXT, N'C01'), CONVERT(TEXT, N'王老师'))
INSERT [dbo].[Teacher] ([TID], [Tname]) VALUES (CONVERT(TEXT, N'C02'), CONVERT(TEXT, N'朱老师'))
INSERT [dbo].[Teacher] ([TID], [Tname]) VALUES (CONVERT(TEXT, N'C03'), CONVERT(TEXT, N'李老师'))
INSERT [dbo].[Teacher] ([TID], [Tname]) VALUES (CONVERT(TEXT, N'C04'), CONVERT(TEXT, N'张老师'))
INSERT [dbo].[Teacher] ([TID], [Tname]) VALUES (CONVERT(TEXT, N'C05'), CONVERT(TEXT, N'陈老师'))
INSERT [dbo].[Teacher] ([TID], [Tname]) VALUES (CONVERT(TEXT, N'C06'), CONVERT(TEXT, N'杨老师'))
INSERT [dbo].[Teacher] ([TID], [Tname]) VALUES (CONVERT(TEXT, N'C07'), CONVERT(TEXT, N'马老师'))
/****** Object:  Table [dbo].[Student]    Script Date: 01/14/2013 22:30:24 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[Student](
    [SID] [int] NOT NULL,
    [Sname] [varchar](10) NULL,
    [Ssex] [varchar](5) NULL,
    [Sage] [int] NULL,
 CONSTRAINT [PK_Student] PRIMARY KEY CLUSTERED 
(
    [SID] ASC
) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
INSERT [dbo].[Student] ([SID], [Sname], [Ssex], [Sage]) VALUES (1001, CONVERT(TEXT, N'猫猫'), CONVERT(TEXT, N'男'), 20)
INSERT [dbo].[Student] ([SID], [Sname], [Ssex], [Sage]) VALUES (1002, CONVERT(TEXT, N'狗狗'), CONVERT(TEXT, N'男'), 19)
INSERT [dbo].[Student] ([SID], [Sname], [Ssex], [Sage]) VALUES (1003, CONVERT(TEXT, N'猪猪'), CONVERT(TEXT, N'女'), 19)
INSERT [dbo].[Student] ([SID], [Sname], [Ssex], [Sage]) VALUES (1004, CONVERT(TEXT, N'小鸟'), CONVERT(TEXT, N'男'), 21)
/****** Object:  Table [dbo].[SC]    Script Date: 01/14/2013 22:30:24 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[SC](
    [SID] [int] NULL,
    [CID] [varchar](10) NULL,
    [Score] [int] NULL
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1004, CONVERT(TEXT, N'002'), 13)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1004, CONVERT(TEXT, N'004'), 13)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1004, CONVERT(TEXT, N'005'), 13)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1004, CONVERT(TEXT, N'006'), 13)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1004, CONVERT(TEXT, N'001'), 13)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1004, CONVERT(TEXT, N'003'), 35)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1003, CONVERT(TEXT, N'002'), 95)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1003, CONVERT(TEXT, N'001'), 55)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1001, CONVERT(TEXT, N'006'), 55)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1001, CONVERT(TEXT, N'005'), 100)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1001, CONVERT(TEXT, N'001'), 100)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1002, CONVERT(TEXT, N'001'), NULL)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1001, CONVERT(TEXT, N'002'), 90)
INSERT [dbo].[SC] ([SID], [CID], [Score]) VALUES (1003, CONVERT(TEXT, N'003'), 68)
/****** Object:  Table [dbo].[Course]    Script Date: 01/14/2013 22:30:24 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[Course](
    [CID] [varchar](10) NOT NULL,
    [Cname] [varchar](10) NULL,
    [TID] [varchar](10) NULL,
 CONSTRAINT [PK_Course] PRIMARY KEY CLUSTERED 
(
    [CID] ASC
) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
INSERT [dbo].[Course] ([CID], [Cname], [TID]) VALUES (CONVERT(TEXT, N'001'), CONVERT(TEXT, N'自然'), CONVERT(TEXT, N'C02'))
INSERT [dbo].[Course] ([CID], [Cname], [TID]) VALUES (CONVERT(TEXT, N'002'), CONVERT(TEXT, N'社会'), CONVERT(TEXT, N'C01'))
INSERT [dbo].[Course] ([CID], [Cname], [TID]) VALUES (CONVERT(TEXT, N'003'), CONVERT(TEXT, N'心理'), CONVERT(TEXT, N'C03'))
INSERT [dbo].[Course] ([CID], [Cname], [TID]) VALUES (CONVERT(TEXT, N'004'), CONVERT(TEXT, N'地理'), CONVERT(TEXT, N'C05'))
INSERT [dbo].[Course] ([CID], [Cname], [TID]) VALUES (CONVERT(TEXT, N'005'), CONVERT(TEXT, N'数学'), CONVERT(TEXT, N'C06'))
INSERT [dbo].[Course] ([CID], [Cname], [TID]) VALUES (CONVERT(TEXT, N'006'), CONVERT(TEXT, N'化学'), CONVERT(TEXT, N'C06'))

 

 
--问题：
--1、查询“”课程比“”课程成绩高的所有学生的学号:
select distinct SID from SC as sc where 
(select score from sc as sc1 where CID='001' and sc1.SID=sc.SID)
 > (select score from sc as sc2 where CID='002' and sc2.SID=sc.SID)
 
select SID from SC s where CID='001' and  score > (select score from sc where CID='002' and s.SID = SID)
 
select a.SID from (select SID,score from SC where CID='001') a,(select SID,score  from SC where CID='002') b
where a.score>b.score and a.SID=b.SID
 
--2、查询平均成绩大于分的同学的学号和平均成绩；
select SID,avg(score) from SC group by SID having avg(score)>60
 
--3 查询所有同学的学号、姓名、选课数、总成绩；
 
select distinct a.SID,b.Sname,
(select  count(CID) as c_total from SC where SID=a.SID) as total_class,
(select sum(Score) from sc  where SID=a.SID) as total_score
from SC as a inner join Student as b on a.SID=b.SID
 
select Student.SID,sname,count(CID),sum(score) from SC join Student on SC.SID =Student.SID group by Student.SID,Student.sname
 
--4 查询姓“杨”的老师的个数；
select count(Tname) from Teacher where Tname like '杨%'
 
--5、查询没学过“杨老师”老师课的同学的学号、姓名；
select SID,Sname from Student  where SID in
(select SID from sc s,course c,teacher t where s.CID=c.CID and c.TID=t.TID and t.tname != '杨老师')
 
--6、查询学过“”并且也学过编号“”课程的同学的学号、姓名；
select SID,Sname from student where SID in
(select SID from SC where CID='001' and SID in (select SID from sc where CID='002'))
 
select s.SID,s.Sname from Student s,SC where SC.SID=s.SID and CID='001' and exists(select * from SC where  CID='002' and SID=s.SID)
select Student.SID,Student.Sname from Student,SC where Student.SID=SC.SID and SC.CID='001'and exists( Select * from SC as SC_2 where SC_2.SID=SC.SID and SC_2.CID='002');
 
 
--7、查询学过“杨老师”老师所教的所有课的同学的学号、姓名；
select SID,Sname from student where SID in
(select SID from SC s,course c,teacher t where s.CID=c.CID and c.TID=t.TID and t.tname='杨老师')
 
select SID,sname 
from student 
where SID in(select s.SID from Course c,sc s,teacher t where t.TID = c.TID and c.CID=s.CID and t.Tname='杨老师')

select SID,Sname
from Student
where SID in (select SID from SC ,Course ,Teacher where SC.CID=Course.CID and Teacher.TID=Course.TID and Teacher.Tname='杨老师' group by SID having count(SC.CID)=(select count(CID) from Course,Teacher  where Teacher.TID=Course.TID and Tname='杨老师'));
 
 
--8、查询课程编号“”的成绩比课程编号“”课程低的所有同学的学号、姓名；
select SID,sname from student where SID in
(select SID from sc as a where (select score from sc where CID='001' and SID=a.SID)>(select score from sc where CID='002' and SID=a.SID))
 
select sname,s.SID from Student s,SC where s.SID = sc.SID and CID='001' and score>(select score from sc where CID='002' and SID=s.SID)
 
Select SID,Sname 
from (select Student.SID,Student.Sname,score ,(select score from SC SC_2 where SC_2.SID=Student.SID and SC_2.CID='002') score2
  from Student,SC where Student.SID=SC.SID and CID='001') S_2 
where score2 <score;
 
--9、查询所有课程成绩小于分的同学的学号、姓名；
 select Student.SID,Sname from Student,SC  where Student.SID=SC.SID group by  Student.SID,Sname  having avg(score)<60
select SID,Sname from Student where SID not in (select Student.SID from Student s,SC where S.SID=SC.SID and score>60);
 
--10、查询没有学全所有课的同学的学号、姓名；
    select Student.SID,Student.Sname
    from Student,SC
    where Student.SID=SC.SID group by  Student.SID,Student.Sname having count(CID) <(select count(CID) from Course);
    
--11、查询至少有一门课与学号为“”的同学所学相同的同学的学号和姓名；
    select Student.SID,Sname from Student,SC where Student.SID=SC.SID and CID in (select CID from SC where SID='1001');
    
--12、查询至少学过学号为“”同学所有一门课的其他同学学号和姓名；
    select distinct SC.SID,Sname
    from Student,SC
    where Student.SID=SC.SID and CID in (select CID from SC where SID='1001');
    
--13、把“SC”表中“杨老师”老师教的课的成绩都更改为此课程的平均成绩；
    update SC set score = (select avg(SC_2.score) from SC SC_2 where SC_2.CID=SC.CID ) 
    from Course,Teacher 
    where Course.CID=SC.CID and Course.TID=Teacher.TID and Teacher.Tname='杨老师';
    
--14、查询和“”号的同学学习的课程完全相同的其他同学学号和姓名；
    select SID from SC where CID in (select CID from SC where SID='1002')
    group by SID having count(*)=(select count(*) from SC where SID='1002');
    
--15、删除学习“杨老师”老师课的SC表记录；
    Delete SC
    from course ,Teacher 
    where Course.CID=SC.CID and Course.TID= Teacher.TID and Tname='杨老师';
    
--16、向SC表中插入一些记录，这些记录要求符合以下条件：没有上过编号“”课程的同学学号、、    号课的平均成绩；
    Insert SC select SID,'002',(Select avg(score)
    from SC where CID='002') from Student where SID not in (Select SID from SC where CID='002');
    
--17、按平均成绩从高到低显示所有学生的“数据库”、“企业管理”、“英语”三门的课程成绩，
--按如下形式显示：学生ID,,数据库,企业管理,英语,有效课程数,有效平均分

    SELECT SID as 学生ID
        ,(SELECT score FROM SC WHERE SC.SID=t.SID AND CID='004') AS 数据库
        ,(SELECT score FROM SC WHERE SC.SID=t.SID AND CID='001') AS 企业管理
        ,(SELECT score FROM SC WHERE SC.SID=t.SID AND CID='006') AS 英语
        ,COUNT(*) AS 有效课程数, AVG(t.score) AS 平均成绩
    FROM SC AS t
    GROUP BY SID
    ORDER BY avg(t.score) 
    
--18、查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分
    SELECT L.CID As 课程ID,L.score AS 最高分,R.score AS 最低分
    FROM SC L ,SC AS R
    WHERE L.CID = R.CID and
        L.score = (SELECT MAX(IL.score)
                      FROM SC AS IL,Student AS IM
                      WHERE L.CID = IL.CID and IM.SID=IL.SID
                      GROUP BY IL.CID)
        AND
        R.Score = (SELECT MIN(IR.score)
                      FROM SC AS IR
                      WHERE R.CID = IR.CID
                  GROUP BY IR.CID
                    );
                    
--19、按各科平均成绩从低到高和及格率的百分数从高到低顺序
    SELECT t.CID AS 课程号,max(course.Cname)AS 课程名,isnull(AVG(score),0) AS 平均成绩
        ,100 * SUM(CASE WHEN  isnull(score,0)>=60 THEN 1 ELSE 0 END)/COUNT(*) AS 及格百分数
    FROM SC T,Course
    where t.CID=course.CID
    GROUP BY t.CID
    ORDER BY 100 * SUM(CASE WHEN  isnull(score,0)>=60 THEN 1 ELSE 0 END)/COUNT(*) DESC
    
--20、查询如下课程平均成绩和及格率的百分数(用"1行"显示): 企业管理（），马克思（），OO&UML （），数据库（）
    SELECT SUM(CASE WHEN CID ='001' THEN score ELSE 0 END)/SUM(CASE CID WHEN '001' THEN 1 ELSE 0 END) AS 企业管理平均分
        ,100 * SUM(CASE WHEN CID = '001' AND score >= 60 THEN 1 ELSE 0 END)/SUM(CASE WHEN CID = '001' THEN 1 ELSE 0 END) AS 企业管理及格百分数
        ,SUM(CASE WHEN CID = '002' THEN score ELSE 0 END)/SUM(CASE CID WHEN '002' THEN 1 ELSE 0 END) AS 马克思平均分
        ,100 * SUM(CASE WHEN CID = '002' AND score >= 60 THEN 1 ELSE 0 END)/SUM(CASE WHEN CID = '002' THEN 1 ELSE 0 END) AS 马克思及格百分数
        ,SUM(CASE WHEN CID = '003' THEN score ELSE 0 END)/SUM(CASE CID WHEN '003' THEN 1 ELSE 0 END) AS UML平均分
        ,100 * SUM(CASE WHEN CID = '003' AND score >= 60 THEN 1 ELSE 0 END)/SUM(CASE WHEN CID = '003' THEN 1 ELSE 0 END) AS UML及格百分数
        ,SUM(CASE WHEN CID = '004' THEN score ELSE 0 END)/SUM(CASE CID WHEN '004' THEN 1 ELSE 0 END) AS 数据库平均分
        ,100 * SUM(CASE WHEN CID = '004' AND score >= 60 THEN 1 ELSE 0 END)/SUM(CASE WHEN CID = '004' THEN 1 ELSE 0 END) AS 数据库及格百分数
  FROM SC
  
--21、查询不同老师所教不同课程平均分从高到低显示
  SELECT max(Z.TID) AS 教师ID,MAX(Z.Tname) AS 教师姓名,C.CID AS 课程ＩＤ,MAX(C.Cname) AS 课程名称,AVG(Score) AS 平均成绩
    FROM SC AS T,Course AS C ,Teacher AS Z
    where T.CID=C.CID and C.TID=Z.TID
  GROUP BY C.CID
  ORDER BY AVG(Score) DESC
  
--22、查询如下课程成绩第3 名到第6 名的学生成绩单：企业管理（），马克思（），UML （），数据库（）   
--[学生ID],[学生姓名],企业管理,马克思,UML,数据库,平均成绩
    SELECT  DISTINCT top 3
      SC.SID As 学生学号,
        Student.Sname AS 学生姓名,
      T1.score AS 企业管理,
      T2.score AS 马克思,
      T3.score AS UML,
      T4.score AS 数据库,
      ISNULL(T1.score,0) + ISNULL(T2.score,0) + ISNULL(T3.score,0) + ISNULL(T4.score,0) as 总分
      FROM Student,SC  LEFT JOIN SC AS T1
                      ON SC.SID = T1.SID AND T1.CID = '001'
            LEFT JOIN SC AS T2
                      ON SC.SID = T2.SID AND T2.CID = '002'
            LEFT JOIN SC AS T3
                      ON SC.SID = T3.SID AND T3.CID = '003'
            LEFT JOIN SC AS T4
                      ON SC.SID = T4.SID AND T4.CID = '004'
      WHERE student.SID=SC.SID and
      ISNULL(T1.score,0) + ISNULL(T2.score,0) + ISNULL(T3.score,0) + ISNULL(T4.score,0)
      NOT IN
      (SELECT
            DISTINCT
            TOP 15 WITH TIES
            ISNULL(T1.score,0) + ISNULL(T2.score,0) + ISNULL(T3.score,0) + ISNULL(T4.score,0)
      FROM sc
            LEFT JOIN sc AS T1
                      ON sc.SID = T1.SID AND T1.CID = 'k1'
            LEFT JOIN sc AS T2
                      ON sc.SID = T2.SID AND T2.CID = 'k2'
            LEFT JOIN sc AS T3
                      ON sc.SID = T3.SID AND T3.CID = 'k3'
            LEFT JOIN sc AS T4
                      ON sc.SID = T4.SID AND T4.CID = 'k4'
      ORDER BY ISNULL(T1.score,0) + ISNULL(T2.score,0) + ISNULL(T3.score,0) + ISNULL(T4.score,0) DESC);
 
--23、统计列印各科成绩,各分数段人数:课程ID,课程名称,[100-85],[85-70],[70-60],[ <60]
    SELECT SC.CID as 课程ID, Cname as 课程名称
        ,SUM(CASE WHEN score BETWEEN 85 AND 100 THEN 1 ELSE 0 END) AS [100 - 85]
        ,SUM(CASE WHEN score BETWEEN 70 AND 85 THEN 1 ELSE 0 END) AS [85 - 70]
        ,SUM(CASE WHEN score BETWEEN 60 AND 70 THEN 1 ELSE 0 END) AS [70 - 60]
        ,SUM(CASE WHEN score < 60 THEN 1 ELSE 0 END) AS [60 -]
    FROM SC,Course
    where SC.CID=Course.CID
    GROUP BY SC.CID,Cname;
 
--24、查询学生平均成绩及其名次
      SELECT 1+(SELECT COUNT( distinct 平均成绩)
              FROM (SELECT SID,AVG(score) AS 平均成绩
                      FROM SC
                  GROUP BY SID
                  ) AS T1
            WHERE 平均成绩> T2.平均成绩) as 名次,
      SID as 学生学号,平均成绩
    FROM (SELECT SID,AVG(score) 平均成绩
            FROM SC
        GROUP BY SID
        ) AS T2
    ORDER BY 平均成绩 desc;
 
--25、查询各科成绩前三名的记录:(不考虑成绩并列情况)
      SELECT t1.SID as 学生ID,t1.CID as 课程ID,Score as 分数
      FROM SC t1
      WHERE score IN (SELECT TOP 3 score
              FROM SC
              WHERE t1.CID= CID
            ORDER BY score DESC
              )
      ORDER BY t1.CID;
      
--26、查询每门课程被选修的学生数
  select CID,count(SID) from sc group by CID;
  
--27、查询出只选修了一门课程的全部学生的学号和姓名
  select SC.SID,Student.Sname,count(CID) AS 选课数
  from SC ,Student
  where SC.SID=Student.SID group by SC.SID ,Student.Sname having count(CID)=1;
--28、查询男生、女生人数
    Select count(Ssex) as 男生人数 from Student group by Ssex having Ssex='男'
    Select count(Ssex) as 女生人数 from Student group by Ssex having Ssex='女'
--29、查询姓“张”的学生名单
    SELECT Sname FROM Student WHERE Sname like '张%';
--30、查询同名同性学生名单，并统计同名人数
  select Sname,count(*) from Student group by Sname having  count(*)>1;;
--31、年出生的学生名单(注：Student表中Sage列的类型是datetime)
    select Sname,  CONVERT(char (11),DATEPART(year,Sage)) as age
    from student
    where  CONVERT(char(11),DATEPART(year,Sage))='1981';
--32、查询每门课程的平均成绩，结果按平均成绩升序排列，平均成绩相同时，按课程号降序排列
    Select CID,Avg(score) from SC group by CID order by Avg(score),CID DESC ;
--33、查询平均成绩大于的所有学生的学号、姓名和平均成绩
    select Sname,SC.SID ,avg(score)
    from Student,SC
    where Student.SID=SC.SID group by SC.SID,Sname having    avg(score)>85;
--34、查询课程名称为“数据库”，且分数低于的学生姓名和分数
    Select Sname,isnull(score,0)
    from Student,SC,Course
    where SC.SID=Student.SID and SC.CID=Course.CID and  Course.Cname='数据库'and score <60;
--35、查询所有学生的选课情况；
    SELECT SC.SID,SC.CID,Sname,Cname
    FROM SC,Student,Course
    where SC.SID=Student.SID and SC.CID=Course.CID ;
--36、查询任何一门课程成绩在分以上的姓名、课程名称和分数；
    SELECT  distinct student.SID,student.Sname,SC.CID,SC.score
    FROM student,Sc
    WHERE SC.score>=70 AND SC.SID=student.SID;
--37、查询不及格的课程，并按课程号从大到小排列
    select CID from sc where score < 60 order by CID ;
--38、查询课程编号为且课程成绩在分以上的学生的学号和姓名；
    select SC.SID,Student.Sname from SC,Student where SC.SID=Student.SID and Score>80 and CID='003';
--39、求选了课程的学生人数
    select count(*) from sc;
--40、查询选修“杨老师”老师所授课程的学生中，成绩最高的学生姓名及其成绩
    select Student.Sname,score
    from Student,SC,Course C,Teacher
    where Student.SID=SC.SID and SC.CID=C.CID and C.TID=Teacher.TID and Teacher.Tname='杨老师' and SC.score=(select max(score)from SC where CID=C.CID );
--41、查询各个课程及相应的选修人数
    select count(*) from sc group by CID;
--42、查询不同课程成绩相同的学生的学号、课程号、学生成绩
  select distinct  A.SID,B.score from SC A  ,SC B where A.Score=B.Score and A.CID <>B.CID ;
--43、查询每门功成绩最好的前两名
    SELECT t1.SID as 学生ID,t1.CID as 课程ID,Score as 分数
      FROM SC t1
      WHERE score IN (SELECT TOP 2 score
              FROM SC
              WHERE t1.CID= CID
            ORDER BY score DESC
              )
      ORDER BY t1.CID;
--44、统计每门课程的学生选修人数（超过人的课程才统计）。要求输出课程号和选修人数，
--    查询结果按人数降序排列，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  CID as 课程号,count(*) as 人数
    from  sc 
    group  by  CID
    order  by  count(*) desc,CID 
--45、检索至少选修两门课程的学生学号
    select  SID 
    from  sc 
    group  by  SID
    having  count(*)  >  =  2
--46、查询全部学生都选修的课程的课程号和课程名
    select  CID,Cname 
    from  Course 
    where  CID  in  (select  CID  from  sc group  by  CID) 
--47、查询没学过“杨老师”老师讲授的任一门课程的学生姓名
    select Sname from Student where SID not in (select SID from Course,Teacher,SC where Course.TID=Teacher.TID and SC.CID=course.CID and Tname='杨老师');
--48、查询两门以上不及格课程的同学的学号及其平均成绩
    select SID,avg(isnull(score,0)) from SC where SID in (select SID from SC where score <60 group by SID having count(*)>2)group by SID;
--49、检索“”课程分数小于，按分数降序排列的同学学号
    select SID from SC where CID='004'and score <60 order by score desc;
--50、删除“”同学的“”课程的成绩
delete from Sc where SID='001'and CID='001';