数据库sql基础题

Student(S#,Sname,Sage,Ssex) 学生表
Course(C#,Cname,T#) 课程表
SC(S#,C#,score) 成绩表
Teacher(T#,Tname) 教师表

1、查询“001”课程比“002”课程成绩高的所有学生的学号；
  select a.S# from (select s#,score from SC where C#='001') a,

  (select s#,score from SC where C#='002') b
  where a.score>b.score and a.s#=b.s#;

  ----解法2----

  select a.id from student a, sc b, sc c where b.cid=1 and c.cid=2 and a.id=b.sid and a.id=c.sid and b.score>c.score

点评：需构造两个集合，一个001课程的集合， 一个002课程的集合

 

2、查询平均成绩大于60分的同学的学号和平均成绩；
    select S#,avg(score)
    from sc
    group by S# having avg(score) >60;

 

 

3、查询所有同学的学号、姓名、选课数、总成绩；
  select Student.S#,Student.Sname,count(SC.C#),sum(score)
  from Student left Outer join SC on Student.S#=SC.S#
  group by Student.S#,Sname
点评：因为要查出所有学生的信息， 所以要用右外连接，否则如果一个学生没有选课，则会丢失该学生的信息

 

4、查询没学过“叶平”老师课的同学的学号、姓名； 
    select Student.S#,Student.Sname
    from Student 
    where S# not in (select distinct( SC.S#) from SC,Course,Teacher where  SC.C#=Course.C# and Teacher.T#=Course.T# and Teacher.Tname='叶平');
点评：先找出选修过“叶平”的课程的学生

 

5、查询学过“001”并且也学过编号“002”课程的同学的学号、姓名；

    select a.id from student a, sc b, sc c
    where a.id = b.sid and a.id = c.sid and b.cid='001' and c.cid='002'
点评：做一个sc的自连接是关键

 

6、查询学过“叶平”老师所教的所有课的同学的学号、姓名；
select a.id, count(b.cid) from student a, sc b, course c, teacher d
where a.id = b.sid
and b.cid = c.id
and c.tid = d.id
and d.name='叶平'
group by a.id
having count(b.cid)=(select count(a.id) from course a, teacher b where a.tid=b.id and b.name='叶平')

点评：查出叶平的课程数，查出学生的选了叶平老师的课程数。
7、查询所有课程成绩小于60分的同学的学号；

select id from student a
where a.id not in
(select a.id from student a, sc b where a.id=b.sid and b.score>60)

点评：先找出有选课大于60分的学生

8、查询至少学过学号为“001”同学所有一门课的其他同学学号和姓名；
    select distinct SC.S#,Sname
    from Student,SC
    where Student.S#=SC.S# and C# in (select C# from SC where S#='001');

 

 
9、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.C# 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.C#=Course.C#
    GROUP BY SC.C#,Cname; 

 

10、查询各科成绩的及格率
    select a.name, (sum(case when b.score>70 then 1 else 0 end))/(count(b.cid)) as '及格率'
    from course a, sc b where a.id = b.cid group by a.name

11、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 a.name, avg(b.score) from course a, sc b where a.id = b.cid group by a.name order by avg(b.score) asc, a.name asc

12、查询至少选修了两门课程的学生学号
    select a.id from student a, sc b where a.id = b.sid  group by a.id having count(b.cid)>=2

13、怎么优化sql语句以提高查询速度

    1 减少数据库访问次数;

    2 select语句避免使用select *, 因为数据库把*转换成各自段名称需要查询数据字典, 需要耗费更多时间;

    3 如无必要不要使用distinct;

    4 不要使用cursor;