数据库基础二（查询大全）

-- 新建表（学生表，教师表，课程表，成绩表）

/*
create table student(
sid int,
sname varchar(255),
sage int,
ssex varchar(255)
);

create table teacher(
tid int,
tname varchar(255)
);

create table course(
cid varchar(255),
cname varchar(255),
tid int
);

create table sc(
sid int,
cid varchar(255),
score int
);
*/
-- 基础信息录入
/*
insert into student values(1,"张三1",17,"男");
insert into student values(2,"张三2",18,"女");
insert into student values(3,"张三3",16,"男");
insert into student values(4,"张三6",17,"男");
insert into student values(5,"李四1",17,"女");
insert into student values(6,"李四2",18,"男");
insert into student values(7,"李四3",16,"男");
insert into student values(8,"李四6",17,"男");
insert into student values(9,"王五1",17,"女");
insert into student values(10,"王五2",19,"女");
insert into student values(11,"王五2",17,"男");
*/

/*
insert into teacher values(1,"李铎");
insert into teacher values(2,"叶平");
insert into teacher values(3,"邢鹏辉");
insert into teacher values(4,"王波");
*/
/*
insert into course values("001","离散数学",1);
insert into course values("002","线性代数",1);
insert into course values("003","具体数学",3);
insert into course values("004","数值分析",2);
insert into course values("005","高等数学",3);
*/

insert into sc values(1,"001",89);
insert into sc values(1,"002",95);
insert into sc values(1,"003",68);
insert into sc values(1,"004",73);
insert into sc values(1,"005",79);

insert into sc values(2,"001",99);
insert into sc values(2,"002",98);
insert into sc values(2,"003",97);
insert into sc values(2,"004",96);
insert into sc values(2,"005",95);

insert into sc values(3,"001",43);
insert into sc values(3,"002",26);
insert into sc values(3,"003",35);
insert into sc values(3,"004",51);
insert into sc values(3,"005",6);

insert into sc values(4,"001",63);
insert into sc values(4,"002",66);
insert into sc values(4,"003",65);
insert into sc values(4,"004",61);
insert into sc values(4,"005",66);

insert into sc values(5,"001",81);
insert into sc values(5,"002",91);
insert into sc values(5,"003",61);
insert into sc values(5,"004",71);
insert into sc values(5,"005",72);

insert into sc values(6,"001",85);
insert into sc values(6,"002",94);
insert into sc values(6,"003",63);
insert into sc values(6,"004",72);
insert into sc values(6,"005",71);

insert into sc values(7,"001",75);
insert into sc values(7,"002",64);
insert into sc values(7,"003",83);
insert into sc values(7,"004",42);
insert into sc values(7,"005",1);

insert into sc values(8,"001",81);
insert into sc values(8,"002",92);
insert into sc values(8,"003",63);
insert into sc values(8,"004",73);
insert into sc values(8,"005",54);

insert into sc values(9,"001",41);
insert into sc values(9,"002",62);
insert into sc values(9,"003",73);
insert into sc values(9,"004",54);
insert into sc values(9,"005",25);

insert into sc values(10,"001",86);
insert into sc values(10,"002",79);
insert into sc values(10,"003",99);
insert into sc values(10,"004",28);
insert into sc values(10,"005",10);

1、查询（"001"课程比"001"课程成绩高）的所有学生的学号。    

    as起别名，可以给查询的列起别名，子查询结果集必须起别名，as可以省略用“ ”空格代替。

    from后面如果是多张表，用“，”连接

    多个表连接条件where里面直接写就可以

select a.c 学号 
from 
(select sid c ,score from sc where cid = '001') a,
(select sid c, score from sc where cid = '002') b
where a.c = b.c and a.score>b.score;

2.查询平均成绩大于等于60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。

    聚合函数：count（）、sum（）、max()、min（）、avg（）。

    

select student.sid 学号,sname 姓名,count(score) 选课数,sum(score) 成绩总和
from student ,sc
where student.sid=sc.sid
GROUP BY student.sid;

4.查询性“李”的老师的个数。

    通配符 % 和

    %是0~n个任意字符

    _ 是一个字符

    如果长度至少是两个字符，设计为“李_%”

select count(tid) 老师个数 
from teacher
where tname LIKE "李_%";

5.查询没学过“叶平”老师的课程的同学的学号、姓名。

    not in 和 in 描述差集和交集

    问题转化为学过“叶平”老师课程的同学

/*
select sid 
from sc,course,teacher
where sc.cid=course.cid and teacher.tid = course.tid and teacher.tname="叶平"；
*/

select sid,sname 
from student
WHERE sid not in (select sid 
from sc,course,teacher
where sc.cid=course.cid and teacher.tid = course.tid and teacher.tname="叶平");

    附上查询结果：

6.查询所有有挂科的学生和姓名。

    （1）DISTINCT    去重复（不建议使用）

select DISTINCT(sc.sid),sname
from sc,student 
where sc.sid = student.sid and score<60 ;

使用group by 分组聚合

select sc.sid,sname
from sc,student 
where sc.sid = student.sid and score<60 GROUP BY student.sid ;

7.查询至少一门课程与学生学号为“2”的同学所学课程相同的学生学号和姓名

    （1）求学号为2同学课程cid集合

select cid 
from sc
where sc.sid=2;

select student.sid,sname
from student,sc
where student.sid=sc.sid and cid in(select cid from sc where sc.sid=2) GROUP BY student.sid HAVING student.sid<>2;

8、统计列印各个科目的成绩，各个科目按分数段 课程ID,课程名称，【100-85】，【85-70】，【70-60】，【60-0】

    case语句

    case when  1>2 then 10 else 20 end;

    sum(select case when 1<2 then 1 else 0 end);

    select sum(case when score >=85 and score<=100 then 1 else 0 end)from sc;

select sc.cid 课程ID,cname 课程名称,
sum(case when score>=85 and score<=100 then 1 else 0 end) as '[85-100]',
sum(case when score>=75 and score<85 then 1 else 0 end) as '[75-85]',
sum(case when score>=60 and score<75 then 1 else 0 end) as '[60-75]',
sum(case when score>=0 and score<60 then 1 else 0 end) as '[0-60]'
from course,sc
where course.cid = sc.cid 
GROUP BY sc.cid;

    查询结果如下：

9.查询每门课程被选修的学生数。

select sc.cid,cname,count(sc.cid) as'总数'
from sc,course
where sc.cid = course.cid
group by sc.cid;

10.查询出只选修了一门课程的学生的学号、姓名。

select student.sid,sname
from student,sc
where student.sid = sc.sid
GROUP BY sc.sid
having count(*)=1;

11.查询男生、女生的总人数。

    性别   人数

    男生   3

    女生   4

#第一种
select ssex ,count(*)as "人数"
from student 
GROUP BY ssex;

#第二种
select case when ssex='男' then '男生' else '女生' end as "性别",count(*) as '人数'
from student
GROUP BY ssex;

#第三种
(select "男生" as "性别",count(*) as "人数" from student where ssex='男')
UNION
(select "女生" as "性别",count(*) as "人数" from student where ssex='女')

12.姓李的师生名单。

(select sname as '姓名' from student where sname like '李_%')
UNION
(select tname as '姓名' from teacher where tname like '李_%');

13.查询出同名同姓的学生名单，并统计人数。

select sname ,count(*)
from student 
GROUP BY sname
having count(*)>1;

14.查询每门课程的平均成绩，结果按平均成绩升序排列，如果平均成绩相同，则按课程号降序排列。

select cid,avg(score)
from sc
GROUP BY cid
ORDER BY avg(score), cid desc;

15、查询平均成绩大于85的所有的学生的学号、姓名、平均成绩.

select student.sid,sname,avg(score)
from student,sc
where student.sid=sc.sid
GROUP BY student.sid
having avg(score)>85;

16、查询课程名称为“离散数学”，且分数低于60分的学生姓名和分数

select student.sname,score
from student,sc,course
where student.sid=sc.sid and sc.cid = course.cid and score<60 and course.cname="离散数学"

17、查询所有学生的选课情况：包括学号、课程号、姓名、课程名。

select student.sid 学号,course.cid 课程号,sname 姓名,cname 课程名
from student,sc,course
where student.sid=sc.sid and sc.cid = course.cid ;

 18、查询有学生不及格的课程的课程号和课程名，并按照课程号从大到小排列.

select sc.cid,cname
from course,sc
where  course.cid=sc.cid and score<60
GROUP BY sc.cid
ORDER BY cid desc;

19、查询课程号为003且成绩在80分以上的学生的学号、姓名.

select student.sid,sname
from student,sc
where student.sid=sc.sid and score>80 and sc.cid="003";

20、查询各个老师相应的学生总人数.

select teacher.tname,count(DISTINCT(sc.sid))
from teacher,sc,course
where teacher.tid = course.tid and course.cid = sc.cid
group by teacher.tid; 

 21、查询不同课程成绩相同的学生的学号、课程号、学生成绩(自连接)。

select a.sid,a.cid,a.score
from sc a ,sc b
where a.score = b.score and a.cid<>b.cid and a.sid=b.sid;

22、查询选“001”课程的所有学生的平均分（去掉一个最高分，去掉一个最低分）

select (sum(score)-max(score)-min(score))/(count(score)-1)
from sc
where cid="001";

23.关于join使用外连接和内连接

    （1）左连接

select *
from teacher left JOIN course on teacher.tid=course.tid;

    （2）右连接

select *
from course RIGHT JOIN teacher ON   teacher.tid = course.tid;