数据库查询

目录

多表查询

笛卡尔积

连接查询

集合

子查询

单行

null

多值

多行

加一条重复数据

找出表中重复数据

表格的操作

创建与查看

修改表格

对数据的增删改

44条练习

-- 1、查询“c001”课程比“c002”课程成绩高的所有学生的学号；

-- 2、查询平均成绩大于60 分的同学的学号和平均成绩；

-- 3、查询所有同学的学号、姓名、选课数、总成绩；

-- 4、查询姓“刘”的老师的个数；

-- 5、查询没学过“谌燕”老师课的同学的学号、姓名；

-- 6、查询学过“c001”并且也学过编号“c002”课程的同学的学号、姓名；

-- 7、查询学过“谌燕”老师所教的所有课的同学的学号、姓名；

-- 8、查询课程编号“c002”的成绩比课程编号“c001”课程低的所有同学的学号、姓名；

-- 9、查询所有课程成绩小于60 分的同学的学号、姓名；

-- 10、查询没有学全所有课的同学的学号、姓名；

-- 11、查询至少有一门课与学号为“s001”的同学所学相同的同学的学号和姓名；

-- 12、把“SC”表中“谌燕”老师教的课的成绩都更改为此课程的平均成绩；-- 相关子查询    用父表的数据 逐条 遍历子表的数据 

-- 13、查询和“s001”号的同学学习的课程完全相同的其他同学学号和姓名；

-- 14、删除学习“谌燕”老师课的SC 表记录；

-- 15、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“c002”课程的同学学号、“c002”号课的平均成绩；

-- 16、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

-- 17、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

-- 18、查询不同老师所教不同课程平均分从高到低显示

-- 19、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]

-- 20、查询各科成绩前三名的记录:(不考虑成绩并列情况)

-- 21、查询每门课程被选修的学生数

-- 22、查询出只选修了一门课程的全部学生的学号和姓名

-- 23、查询男生、女生人数

-- 24、查询姓“张”的学生名单

-- 25、查询同名同性学生名单，并统计同名人数

-- 26、1981 年出生的学生名单(注：Student 表中Sage 列的类型是number)

-- 27、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

-- 28、查询平均成绩大于85 的所有学生的学号、姓名和平均成绩

-- 29、查询课程名称为“数据库”，且分数低于60 的学生姓名和分数

-- 30、查询所有学生的选课情况；

-- 31、查询任何一门课程成绩在70 分以上的姓名、课程名称和分数；

-- 32、查询不及格的课程，并按课程号从大到小排列

-- 33、查询课程编号为c001 且课程成绩在80 分以上的学生的学号和姓名；

-- 34、求选了课程的学生人数

-- 35、查询选修“谌燕”老师所授课程的学生中，成绩最高的学生姓名及其成绩

-- 36、查询各个课程及相应的选修人数

-- 37、查询不同课程成绩相同的学生的学号、课程号、学生成绩

-- 38、查询每门功课成绩最好的前两名

-- 39、统计每门课程的学生选修人数（超过10 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，若人数相同，按课程号升序排列

-- 40、检索至少选修两门课程的学生学号

-- 41、查询全部学生都选修的课程的课程号和课程名

-- 42、查询两门以上不及格课程的同学的学号及其平均成绩

-- 43、检索“c004”课程分数小于60，按分数降序排列的同学学号

-- 44、删除“s002”同学的“c001”课程的成绩

---

多表查询

笛卡尔积

将两个表拼接在一起，会有n*m个数据，运用笛卡尔积把相匹配的联系起来

--笛卡尔积
SELECT * FROM emp e,dept d；

--等值连接
SELECT * FROM emp e,dept d where e.deptno=d.deptno; 

--非等值连接
SELECT * FROM emp e,salgrade s where e.sal between s.losal and s.hisal;

连接查询

--自连接   找出每个员工和对应的领导  把自身认为是两张表 
select e.ename,m.ename from emp e,emp m where e.mgr =m.empno; --会有一个没有领导就不会显示

--左外连接   没有领导的也会显示 保留左边
select  e.ename,m.ename from emp e left join emp m on e.mgr =m.empno;

--右外连接  以右边的为主，尽管左边的没有
select  e.ename,m.ename from emp e right join emp m on e.mgr =m.empno;

--满外连接 保留两边全部，尽管没匹配到
SELECT * FROM emp e full outer join dept d on e.deptno=d.deptno;

通过group by 分组时只能select 分组的依据和计算聚合的结果

SELECT count(ename),
d.dname
FROM emp e
LEFT JOIN dept d
ON e.deptno=d.deptno
GROUP BY d.dname;


--但不能
SELECT count(ename),
d.deptno
FROM emp e
LEFT JOIN dept d
ON e.deptno=d.deptno
GROUP BY d.dname;

集合

--union 并集，所有内容都查询，重复的只显示一次
语句A
union
语句B

--union all 并集 所有都显示


--nimus  差集

--intersect 交集

子查询

单行

select * from emp where sal > ( select sal from emp where empno =7566);

null

如果子查询结果是Null则结果也是null

多值

--找出部门10工资都高的人
 select * from emp where sal > (select max(sal) from emp where deptno=10);--单行
 select * from emp where sal > all (select sal from emp where deptno=10);

--找出任意一个比部门10工资高的人
 select * from emp where sal > (select min(sal) from emp where deptno=10);--单行
 select * from emp where sal >  any(select sal from emp where deptno=10);

--找出一个与部门10工资相同

select * from emp where sal in (select sal from emp where deptno=30) and (deptno!=30 or deptno is null);

多行

   伪列 rowid :数据存储的物理地址    rownumber:序号，一定从1开始

注意，*与rownum同时选择可能会出现问题

select e.*,rowid,rownum from emp e;

--如果结果中没有1则不会出现结果
--选择工资前三的人
select t.empno,t.sal,t.rid,t.rno,rownum from
(select e.*, rowid rid,rownum rno from emp e order by sal desc) t
where rownum<=3;

--但要让选择工资从第三到第五，只能进行再次子查询
select p.*,rownum from
(select t.empno,t.sal,rownum ru from
(select e.* from emp e order by sal desc) t
where rownum<=5) p where p.ru>=3;

加一条重复数据

insert into emp
select 8002,e.ename,e.job,e.mgr,e.hiredate,e.sal,e.comm,e.deptno
from emp e where e.empno =7844;
commit;

找出表中重复数据

1.对重复字段进行分组

2.把区分特征放到select 里

SELECT * FROM emp t where t.empno not in 
(select min( e.empno) from emp e group by e.ename,e.job,e.mgr,e.hiredate,e.sal,e.comm,e.deptno); 

如果都找不到其他区分特征，则rowid是一定可以作为区分特征。

select count(ename),deptno from emp group by deptno;
SELECT * FROM emp;
select count(e.ename),d.dname from emp e left join dept d on e.deptno=d.deptno  group by d.dname ;
select e.ename from emp e where e.sal> (select sal from emp where ename ='JONES');
select t.job,t.asl from 
(select job,avg(sal) asl,rownum ru from emp  group by job order by avg(sal) desc) t
where t.ru<3;
select ename,deptno from emp where sal > all(select sal from emp where deptno=20) and deptno !=20;
select t.job,t.asl from 
(select job,avg(sal) asl from emp group by job) t 
where t.asl>2000;

练习

--列出员工表中每个部门的员工数，和部门 no
select count(ename),deptno from emp group by deptno;

--列出员工表中每个部门的员工数（员工数必须大于 3），和部门名称
select count(e.ename),d.dname from emp e left join dept d on e.deptno=d.deptno  group by d.dname ;

--找出工资比 jones 多的员工
select e.ename from emp e where e.sal> (select sal from emp where ename ='JONES');

--列出所有员工的姓名和其上级的姓名
SELECT e.ename,p.ename FROM emp e,emp p where e.empno=p.mgr;

--以职位分组，找出平均工资最高的两种职位
select job from 
(select job,avg(sal) from emp  group by job order by avg(sal) desc) 
where rownum<3;

--查找出不在部门 20，且比部门 20 中任何一个人工资都高的员工姓名、部门名称
select ename,dname from (select *from emp where sal > all(select sal from emp where deptno=20) and (deptno !=20 and deptno is not null) )t1 join dept t2 on t1.deptno=t2.deptno;

--得到平均工资大于 2000 的工作职种
select job,t.asl from 
(select job,avg(sal) asl from emp group by job) t 
where t.asl>2000;

--分部门得到工资大于 2000 的所有员工的平均工资，并且平均工资还要大于2500
select job,asl from 
(select job,avg(sal) asl from emp group by job) 
where asl>2000 and asl>2500;

--得到每个月工资总数最少的那个部门的部门编号，部门名称，部门位置
select * from
(select t1.deptno,t2.dname,t2.loc from 
(select deptno,avg(sal) asl from emp group by deptno)t1 left join dept t2 on t1.deptno=t2.deptno)
where rownum =1 ;

--分部门得到平均工资等级为 2 级（等级表）的部门编号
select deptno from
 (select deptno,avg(sal) asl from emp  group by deptno) t1,salgrade t2
 where  t2.grade=3 and t1.asl between t2.losal and t2.hisal;

--查找出部门 10 和部门 20 中，工资最高第 3 名到工资第 5 名的员工的员工名字，部门名字，部门位置
select t1.sal,t1.ename,t1.deptno,t2.loc,t1.ru from
(select sal,ename,deptno,rownum ru from 
(select sal,ename,deptno from emp order by sal desc) where rownum<=5) t1 
join dept t2 on t1.deptno=t2.deptno 
where t1.ru>=3;

--查找出收入（工资加上奖金），下级比自己上级还高的员工编号，员工名字，员工收入
select t1.ename,t1.sal+nvl(t1.comm,0),t2.ename,t2.empno,t2.sal+nvl(t2.comm,0) from emp t1,emp t2 where t1.empno=t2.mgr and t1.sal+nvl(t1.comm,0)=2;

--查询出 king 所在部门的部门号\部门名称\部门人数
select t.deptno,d.dname,t.pn from
(select count(ename) pn,deptno from emp group by deptno having deptno =
(select deptno from emp where ename='KING')) t left join dept d on t.deptno= d.deptno;

--查询出 king 所在部门的工作年限最大的员工名字
SELECT ename FROM emp where sysdate-hiredate=
(SELECT max(sysdate-hiredate) FROM emp 
where deptno=( select deptno from emp where ename='KING'));

--查询出工资成本最高的部门的部门号和部门名称
select t.deptno,d.dname from
(select deptno ,sum(sal+nvl(comm,0)) from emp group by deptno order by sum(sal+nvl(comm,0)) desc ) t left join dept d on t.deptno=d.deptno
where rownum=1;

表格的操作

创建与查看

--创建表
create table student(
sno varchar2(10) primary key, --varchar2 表示可变字符串， primary key主键
sname varchar2(20),
sage number(3),
ssex varchar2(5)
);
commit;--提交

--查看
select * from user_tables;

修改表格

--添加列
alter table student add phone varchar2(11)

--删除列，列中不能有数据
alter table student drop column phone;

--修改列
alter table student modify phone int;

--修改表名
alter table student rename to tb_student;

对数据的增删改

--插入
insert into student values ('s001','张三',23,'男');
--用查询结果插
insert into student 
select s0003,name,age from student where sno=2;

--修改
update student set sname='李思' where sno='s001';

--删除 用主键
delete from student where sno = 3;

44条练习

前提

/**********************创建表*************************/
create table student(
  sno varchar2(10) primary key,
  sname varchar2(20),
  sage number(3),
  ssex varchar2(5)
);
create table teacher(
  tno varchar2(10) primary key,
  tname varchar2(20)
);
create table course(
  cno varchar2(10),
  cname varchar2(20),
  tno varchar2(20),
  constraint pk_course primary key (cno,tno)
);
create table sc(
  sno varchar2(10),
  cno varchar2(10),
  score number(4,2),
  constraint pk_sc primary key (sno,cno)
);
/*******初始化学生表的数据******/
insert into student values ('s001','张三',23,'男');
insert into student values ('s002','李四',23,'男');
insert into student values ('s003','吴鹏',25,'男');
insert into student values ('s004','琴沁',20,'女');
insert into student values ('s005','王丽',20,'女');
insert into student values ('s006','李波',21,'男');
insert into student values ('s007','刘玉',21,'男');
insert into student values ('s008','萧蓉',21,'女');
insert into student values ('s009','陈萧晓',23,'女');
insert into student values ('s010','陈美',22,'女');
commit;
/******************初始化教师表***********************/
insert into teacher values ('t001', '刘阳');
insert into teacher values ('t002', '谌燕');
insert into teacher values ('t003', '胡明星');
commit;
/***************初始化课程表****************************/
insert into course values ('c001','J2SE','t002');
insert into course values ('c002','Java Web','t002');
insert into course values ('c003','SSH','t001');
insert into course values ('c004','Oracle','t001');
insert into course values ('c005','SQL SERVER 2005','t003');
insert into course values ('c006','C#','t003');
insert into course values ('c007','JavaScript','t002');
insert into course values ('c008','DIV+CSS','t001');
insert into course values ('c009','PHP','t003');
insert into course values ('c010','EJB3.0','t002');
commit;
/***************初始化成绩表***********************/
insert into sc values ('s001','c001',78.9);
insert into sc values ('s002','c001',80.9);
insert into sc values ('s003','c001',81.9);
insert into sc values ('s004','c001',60.9);
insert into sc values ('s001','c002',82.9);
insert into sc values ('s002','c002',72.9);
insert into sc values ('s003','c002',81.9);
insert into sc values ('s001','c003','59');
commit;

-- 1、查询“c001”课程比“c002”课程成绩高的所有学生的学号；

-- sc 
SELECT t1.sno
FROM sc t1,
  sc t2
WHERE t1.sno = t2.sno
AND t1.cno   = 'c001'
AND t2.cno   = 'c002'
AND t1.score > t2.score;

-- 能用连接查询,尽量使用连接查询
SELECT t1.sno
FROM sc t1
JOIN sc t2
ON t1.sno    = t2.sno
AND t1.cno   = 'c001'
AND t2.cno   = 'c002'
AND t1.score > t2.score;

-- 2、查询平均成绩大于60 分的同学的学号和平均成绩；

-- sc 
SELECT sno,AVG(score) FROM sc GROUP BY sno HAVING AVG(score) > 60;

-- 3、查询所有同学的学号、姓名、选课数、总成绩；

-- student,sc 
-- 以sc 查 sno count(cno) sum(score)
-- 以 sno  和 student 关联
select s.sno,s.sname, nvl(t.count_sno,0),nvl(t.sum_score,0) from student s left join 
(select sno,count(cno) count_sno,sum(score) sum_score from sc group by sno) t on s.sno = t.sno;
-- 直接关联 student,sc 
select s.sno,s.sname,count(sc.cno),nvl(sum(sc.score),0) from student s left join sc 
on s.sno = sc.sno group by s.sno,s.sname order by count(sc.cno) desc;

select s.sno,max(s.sname),count(sc.cno),nvl(sum(sc.score),0) from student s left join sc 
on s.sno = sc.sno group by s.sno order by count(sc.cno) desc;

-- 4、查询姓“刘”的老师的个数；

-- teacher
select count(t.tname) from teacher t where t.tname like '刘%';

-- 5、查询没学过“谌燕”老师课的同学的学号、姓名；

-- teacher student course sc 
select * from student s where s.sno not in
(select distinct sc.sno from teacher t
join course c on t.tno = c.tno and t.tname = '谌燕'
join sc on c.cno = sc.cno);

-- 6、查询学过“c001”并且也学过编号“c002”课程的同学的学号、姓名；

-- student sc 
select s.sno,s.sname from student s join 
(select distinct t1.sno from sc t1 join sc t2 on t1.sno= t2.sno and t1.cno = 'c001' and t2.cno = 'c002') t
on s.sno = t.sno;

select s.sno,s.sname from student s where s.sno in 
(select distinct t1.sno from sc t1 join sc t2 on t1.sno= t2.sno and t1.cno = 'c001' and t2.cno = 'c002');

-- 7、查询学过“谌燕”老师所教的所有课的同学的学号、姓名；

select * from student s where s.sno in
(select distinct sc.sno from teacher t
join course c on t.tno = c.tno and t.tname = '谌燕'
join sc on c.cno = sc.cno);

-- 8、查询课程编号“c002”的成绩比课程编号“c001”课程低的所有同学的学号、姓名；

select t4.sno,t4.sname from
(select t1.sno from
(select * from sc where cno = 'c001') t1 
join (SELECT * FROM sc where cno ='c002') t2 on t1.sno=t2.sno 
where t1.score> t2.score)t3 join student t4 on t3.sno=t4.sno;

-- 9、查询所有课程成绩小于60 分的同学的学号、姓名；

-- sc 
select * from student s1 where s1.sno in
(select s.sno from sc s where s.sno not in
(select distinct sno from sc where score > 60));
update sc set score =50;
rollback;

select * from 
(select sno,count(cno) cou from sc group by sno) t1
join 
(select sno,count(cno) cou from sc where score < 60 group by sno) t2
on t1.sno = t2.sno and t1.cou = t2.cou;

-- 10、查询没有学全所有课的同学的学号、姓名；

select s.sno,max(s.sname ),count(sc.cno) from student s left join sc 
on s.sno = sc.sno  group by s.sno having count(sc.cno)  <
(select count(cno) from course);

-- 11、查询至少有一门课与学号为“s001”的同学所学相同的同学的学号和姓名；

select distinct s.sno,s.sname from student s join sc on s.sno = sc.sno and s.sno != 's001'
and sc.cno in (select sc.cno from sc where sc.sno = 's001');
insert into sc values ('s005','c004',60);
insert into sc values ('s005','c001',60);
rollback;

-- 12、把“SC”表中“谌燕”老师教的课的成绩都更改为此课程的平均成绩；
-- 相关子查询    用父表的数据 逐条 遍历子表的数据 

update sc t1 set t1.score = 
(select avg(sc.score) from teacher t 
join course c on t.tno = c.tno and t.tname = '谌燕'
join sc on c.cno = sc.cno group by sc.cno having t1.cno = sc.cno)
where t1.cno in
(select c.cno from teacher t join course c on t.tno = c.tno and t.tname = '谌燕');
select * from sc where sc.cno in
(select c.cno from teacher t join course c on t.tno = c.tno and t.tname = '谌燕');
rollback;

-- 13、查询和“s001”号的同学学习的课程完全相同的其他同学学号和姓名；

--先找到有哪些课程，然后通过in留下在这些课程里，在对每个学生数数，看是否是这个课程数，但有可能结果会比s001的课程多因为只留下的是符合s001的数，可以minus sc中的cno不在课程里的

(select sno from sc where cno in
(select sc.cno from sc where sc.sno = 's001') and sno != 's001' group by sno 
having count(cno) = (select count(sc.sno) from sc where sc.sno = 's001'))
minus
(select sno from sc where cno not in (select sc.cno from sc where sc.sno = 's001'));
select * from sc order by sno;
delete from sc where sno= 's002' and cno = 'c004';
insert into sc values ('s002','c003',60);
insert into sc values ('s002','c004',60);
rollback;

select t1.sno,t1.cno,t2.sno,t2.cno from 
(select * from sc where sno !='s001') t1
 join
(select * from sc where sno ='s001') t2 
on t1.cno = t2.cno;

select t1.sno from 
(select * from sc where sno !='s001') t1
join
(select * from sc where sno ='s001') t2 
on t1.cno = t2.cno group by t1.sno having count(t2.cno)  =3
minus
(select sno from sc where cno not in (select sc.cno from sc where sc.sno = 's001'));

---可以先剔除所选课程不在s001中，再数数对比
select count(cno),sno from sc where sno not in
(select distinct sno from sc where cno not in
(select cno from sc where sno='s001')) 
group by sno having sno!='s001' and 
count(cno)=(select count(cno) from sc where sno='s001');

-- 14、删除学习“谌燕”老师课的SC 表记录；

delete from sc where sc.cno in 
(select c.cno from course c where c.tno =
(select t.tno from teacher t where t.tname = '谌燕'));
rollback;
select * from sc;

-- 15、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“c002”课程的同学学号、“c002”号课的平均成绩；

insert into sc 
select sno,'c002',(select avg(score) from sc where cno = 'c002') from student where sno not in
(select sno from sc where cno = 'c002');
rollback;

-- 16、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 cno,max(score),min(score) from sc group by cno;

-- 17、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 cno,
  AVG(score),
  SUM(
  CASE
    WHEN score >= 60
    THEN 1
    ELSE 0
  END )/COUNT(score)*100
  || '%'
FROM sc
GROUP BY cno
ORDER BY SUM(
  CASE
    WHEN score >= 60
    THEN 1
    ELSE 0
  END )/COUNT(score) DESC;

-- 18、查询不同老师所教不同课程平均分从高到低显示

select c.tno,sc.cno,avg(score) from course c join sc on c.cno = sc.cno group by c.tno,sc.cno
order by avg(score) desc;

-- 19、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]

--也可以(select count(sno) from sc  where  sc.cno=t1.cno and score between 85 and 100 ),

select c.cno as "课程ID",c.cname as "课程名称",
sum(case when sc.score > 85 then 1 else 0 end ) as "[100-85]",
sum(case when sc.score > 70 and sc.score <=85 then 1 else 0 end ) as "[85-70]",
sum(case when sc.score >= 60 and sc.score <=70 then 1 else 0 end ) as "[70-60]",
sum(case when sc.score <60 then 1 else 0 end ) as "[ <60]" 
from course c join sc on c.cno = sc.cno group by c.cno,c.cname;

-- 20、查询各科成绩前三名的记录:(不考虑成绩并列情况)

-- row_number() over()
select * from sc order by cno;
-- 查询 各科 成绩比自己高的人数 <=2 
-- 相同课  p.score < c.score   count <=2 
select * from sc p where 
(select count(sno) from sc c where p.cno = c.cno and p.score < c.score) <=2 order by cno,score desc;
select * from 
(select  sc.*,row_number () over (partition by cno
                           order by score desc) rn from sc ) t where t.rn <=3 ;
-- 考虑成绩并列情况

-- 21、查询每门课程被选修的学生数

select c.cno,max(c.cname),count(sc.sno) from course c left join sc on c.cno = sc.cno group by c.cno;

-- 22、查询出只选修了一门课程的全部学生的学号和姓名

select s.sno,max(s.sname) from student s join sc on s.sno = sc.sno group by s.sno having count(sc.sno) =1;

-- 23、查询男生、女生人数

select ssex,count(sno) from student group by ssex;

-- 24、查询姓“张”的学生名单
 

select * from student where sname like '张%';

-- 25、查询同名同性学生名单，并统计同名人数

select sname,ssex,count(sname) from student group by sname,ssex;

-- 26、1981 年出生的学生名单(注：Student 表中Sage 列的类型是number)

select * from student where age = extract(year from sysdate)-1981;

-- 27、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 cno,avg(score) from sc group by cno order by avg(score),cno desc;

-- 28、查询平均成绩大于85 的所有学生的学号、姓名和平均成绩

select sc.sno,max(s.sname),avg(sc.score) from sc join student s on sc.sno = s.sno group by sc.sno;

-- 29、查询课程名称为“数据库”，且分数低于60 的学生姓名和分数

select s.sname,sc.score from student s 
join sc on s.sno = sc.sno and sc.score < 60
join course c on sc.cno = c.cno and c.cname = '数据库';

-- 30、查询所有学生的选课情况；

select * from student s
left join sc on s.sno = sc.sno;

-- 31、查询任何一门课程成绩在70 分以上的姓名、课程名称和分数；

select s.sname,sc.score,c.cname from student s 
join  (select sno from sc group by sno having min(score) > 70) t on s.sno = t.sno
join sc on t.sno = sc.sno
join course c on sc.cno = c.cno;

-- 32、查询不及格的课程，并按课程号从大到小排列

select * from sc where score < 60 order by cno desc;

-- 33、查询课程编号为c001 且课程成绩在80 分以上的学生的学号和姓名；

select s.sno,s.sname,sc.score from student s 
join sc on s.sno = sc.sno and sc.score > 80;

-- 34、求选了课程的学生人数

select count(distinct sno) from sc;

-- 35、查询选修“谌燕”老师所授课程的学生中，成绩最高的学生姓名及其成绩

select t.sname,t.score from 
(select * from student s
join sc on s.sno = sc.sno
join course c on c.cno = sc.cno
join teacher t on c.tno = t.tno and t.tname = '谌燕' 
order by sc.score desc) t 
where rownum = 1;

-- 36、查询各个课程及相应的选修人数

select cno,count(sno) from sc group by cno;

-- 37、查询不同课程成绩相同的学生的学号、课程号、学生成绩

select t1.* from sc t1 , sc t2 where t1.cno != t2.cno and t1.score = t2.score and t1.sno = t2.sno;

-- 38、查询每门功课成绩最好的前两名

select * from sc p where 
(select count(sno) from sc c where p.cno = c.cno and p.score < c.score ) <=1; 

-- 39、统计每门课程的学生选修人数（超过10 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，若人数相同，按课程号升序排列

select cno,count(sno) from sc group by cno having count(sno) > 10 order by count(sno) desc ,cno asc;

-- 40、检索至少选修两门课程的学生学号

select sno,count(cno) from sc group by sno having count(cno) >=2;

-- 41、查询全部学生都选修的课程的课程号和课程名

select sc.cno,count(sc.sno),max(c.cname) from course c 
join sc on c.cno = sc.cno group by sc.cno 
having count(sc.sno) = (select count(sno) from student);

-- 42、查询两门以上不及格课程的同学的学号及其平均成绩

select sno,count(cno) from sc where score < 60 group by sno having count(cno)>2;

-- 43、检索“c004”课程分数小于60，按分数降序排列的同学学号

select sno from sc where score <60 and cno = 'c004' order by score asc;

-- 44、删除“s002”同学的“c001”课程的成绩

delete from sc where sno= 's002' and cno = 'c001';
rollback;