原文：https://blog.csdn.net/weixin_40051278/article/details/80592457
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为sc表中的sno和cno创建外键
alter table sc add foreign key(sno) references student(sno);
alter table sc add foreign key(cno) references course(cno);
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a) 插入如下学生记录（学号：95030，姓名：李莉，年龄：18）
insert into student(sno,sname,sage) values('95030','李莉',18);
b) 插入如下选课记录（95030，1）
insert int sc(sno,cno) values('95030','001');
c) 计算机系学生年龄改成20
update student set sage = 20 where sdept = 'cs';
d) 数学系所有学生成绩改成0
update sc set grade = 0 where cno = '002';
e) 把低于总平均成绩的女同学成绩提高5分
f) 修改2号课程的成绩，若成绩小于75分提高5％，成绩大于75时提高
4％（两个语句实现，注意顺序）
update sc set grade=grade+grade*0.05 where cno='002' and grade<75;
update sc set grade=grade+grade*0.04 where cno='002' and grade>75;
g) 删除95030学生信息
delete from student where sno = '95030';
h) 删除SC表中无成绩的记录
delete from sc where grade is null;
i) 删除张娜的选课记录
delete from sc where sno = (select sno from student where sname = '张娜');
j) 删除数学系所有学生选课记录
delete from sc where cno = '002';
k) 删除不及格的学生选课记录
delete from sc where grade < 60;
l) 查询每一门课程成绩都大于等于80分的学生学号、姓名和性别，把值送往另一个已经存在的基本表STU(SNO,SNAME,SSEX)中
insert into stu(sno,sname,ssex)
select student.sno,student.sname,student.ssex
from student,sc
where student.sno=sc.sno and sc.grade>=80;
m) 把所有学生学号和课程号连接追加到新表中
insert into STUsc(sno,cno)
select student.sno,sc.cno from student,sc
where student.sno = sc.sno;
n) 所有学生年龄增1
update student set sage = sage+1;
o) 统计3门以上课程不及格的学生把
相应的学生姓名、系别追加到另外一个表中
insert into stu(sname,sdept) --插入表中
select sname,sdept from student,sc --选择出列名
where --条件
student.sno=(select sno from sc
where grade<60 group by sno having count(*)>3);
查询每个学生及其选课情况；
select sno,cno from sc;
将STUDENT,SC进行右连接
查询有不及格的学生姓名和所在系
select a.sname,a.sdept from student a right join sc b on a.sno = b.sno;
查询所有成绩为优秀（大于90分）的学生姓名
select a.grade,b.sname ,a.sno
from sc a right join student b on a.grade > 90 and a.sno = b.sno;
+-------+--------+-------+
| grade | sname | sno |
+-------+--------+-------+
| NULL | 丽丽 | NULL |
| NULL | 赵海 | NULL |
| 97 | 刘晨 | 08005 |
| 93 | 刘丹丹 | 08006 |
| NULL | 刘立 | NULL |
| NULL | 王江 | NULL |
| NULL | 高晓 | NULL |
| 100 | 张丽 | 08010 |
| NULL | NULL | NULL |
+-------+--------+-------+
select a.grade,b.sname ,a.sno from sc a
join student b on a.grade > 90 and a.sno = b.sno;
+-------+--------+-------+
| grade | sname | sno |
+-------+--------+-------+
| 97 | 刘晨 | 08005 |
| 93 | 刘丹丹 | 08006 |
| 100 | 张丽 | 08010 |
+-------+--------+-------+
查询既选修了2号课程又选修了3号课程的
学生姓名、学号；
select sname,sno from student where sno in(
select sno from sc
where cno = '003'
and
sno in(select sno from sc where cno = '006'));
----或者
select a.sname,a.sno from student a join sc b where a.sno = b.sno and cno = '003' and b.sno in(select sno from sc where cno = '006');
查询和刘晨同一年龄的学生
select sno ,sname from student where sage = (select sage from student where sname = '刘晨');
选修了课程名为“数据库”的学生姓名和年龄
select a.sname,a.sage from student a join sc b on a.sno = b.sno and b.cno = '001
查询其他系比IS系任一学生年龄小的学生名单
select sname,sage from student where sage < any
(select sage from student where sdept = 'is');
查询其他系中比IS系所有学生年龄都小的学生名单
select sname,sage from student where sage < all
(select sage from student where sdept = 'is');
查询选修了全部课程的学生姓名
select sname,sno
from student where sno in(select sno from sc);
查询计算机系学生及其性别是男的学生
select *from student where sdept = 'cs' and ssex = '男';
查询选修课程1的学生集合和选修2号课程学生集合的差集
select sno from sc where cno = '001' and sno not
in(select sno from sc where cno = '002');
查询李丽同学不学的课程的课程号
select cno from course where cno not in(select a.cno from sc a join student b on a.sno = b.sno and b.sname = '李丽');
查询选修了3号课程的学生平均年龄
select avg(sage) from student where sno in(select sno from sc where cno = '003');
求每门课程学生的平均成绩
select avg(grade) from sc group by cno;
统计每门课程的学生选修人数（超过3人的才统计）。要求输出课程号和选修人数，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(*) from sc group by cno having count(sno) > 3;
查询学号比刘晨大，而年龄比他小的学生姓名。
select sname from student where
sno > (select sno from student where sname = '刘晨')
and sage < (select sage from student where sname = '刘晨')；
求年龄大于女同学平均年龄的男同学姓名和年龄
select sname,sage from student where ssex = '男' and
sage > (select avg(sage) from student where ssex = '女');
求年龄大于所有女同学年龄的男同学姓名和年龄
select sname,sage from student where ssex = '男' and
sage > all (select sage from student where ssex = '女');
查询95001和95002两个学生都选修的课程的信息
select *from sc where sno in ('95001','95002');
-----------------------------------------------------
alter table student add test varchar(20); -
alter table student drop test; -
-----------------------------------------------------
为学生表按学号建立唯一索引
mysql> create UNIQUE INDEX stusno ON student(sno);
Query OK, 10 rows affected (0.61 sec)
Records: 10 Duplicates: 0 Warnings: 0
为course表按课程号升序（默认）建立唯一索引
mysql> create UNIQUE INDEX coucno ON course(cno);
Query OK, 7 rows affected (0.38 sec)
Records: 7 Duplicates: 0 Warnings: 0
为sc表按学号升序和课程号降序建立唯一索引
mysql> create UNIQUE INDEX scno ON sc(sno asc,cno desc);
Query OK, 11 rows affected (0.34 sec)
Records: 11 Duplicates: 0 Warnings: 0
//删除索引
drop index scno on sc;
//插入学生信息
insert into student (sno,sname,sage) values('95030','；李莉',18);
a) 查询全体学生的学号和姓名
select sno,sname from student;
b) 查询全体学生的详细记录
select *from student;
c) 查询所有选修过课程的学生学号
select distinct sno from sc ;
d) 查询考试有不及格的学生学号
select sno from sc where grade < 60;
e) 查询不是信息系（IS）、计算机系（CS）的学生性别、年龄、系别
select sname,ssex,sdept from student where sdept not in('is','cs');
f) 查询选修了4号课的学生学号和成绩，结果按成绩降序排列
select sno,grade from sc where cno = '004' order by grade desc;
g) 查询每个课程号和相应的选课人数
select cno,count(cno) as cnonumed from sc group by cno;
h) 查询计算机系（CS）的学生姓名、年龄、系别
select sname,sage,sdept from student where sdept = 'cs';
i) 查询年龄18－20岁的学生学号、姓名、系别、年龄；
select sname,sage,sdept from student where sage between 18 and 20;
j) 查询姓刘的学生情况
select *from student where sname like '刘%';
k) 查询既选修3号课程，又选修6号课程的学生学号和成绩
select sno ,grade from sc
where cno = '003'
and
sno in(select sno from sc where cno = '006');
l) 查询学生的姓名和出生年份（今年2003年）
select sname,sbirthday from student;
m) 查询没有成绩的学生学号和课程号
select sno,cno from sc where grade is null;
n) 查询总成绩大于200分的学生学号
select sno,sum(grade) from sc
group by sno having sum(grade) > 200;
o) 查询每门课程不及格学生人数
select cno,count(sno) from sc
where grade < 90
group by cno;
p) 查询不及格课程超过3门的学生学号
select cno,count(sno) from sc
where grade < 60
group by cno
having count(sno) > 3;
q) 查询年龄在10到19岁之间的学生信息
select *from student
where sage between 10 and 19;
r) 查询全体学生情况，按所在系升序排列，同一个系的学生按年龄降序排列
select *from student order by sdept asc,sage desc;
s) 查询选了1号课程的学生平均成绩
select cno,avg(grade) from sc where cno = '001' group by cno;
+-----+------------+
| cno | avg(grade) |
+-----+------------+
| 001 | 92.6667 |
+-----+------------+
1 row in set (0.00 sec)
mysql> select cno,avg(grade) from sc group by cno having cno = '001';
+-----+------------+
| cno | avg(grade) |
+-----+------------+
| 001 | 92.6667 |
+-----+------------+
t) 查询选了3号课程的学生的最高分
select cno, max(grade)
from sc where cno = '003' group by cno;
u) 查询每个同学的总成绩
select sno,sum(grade)
from sc
group by sno;
补充部分：字段不同，仅仅只是为了展示某种结果而用{
-- 查询选课门数超过2们的学生的平均成绩和选课门数
SELECT M.NAME,平均成绩,COUNTS 选课门数 FROM(SELECT S.NAME,AVG(G.grade) 平均成绩,COUNT(G.kecheng_id) COUNTS FROM t_student S LEFT JOIN t_grade G ON S.ID=G.student_id GROUP BY S.ID)M WHERE M.COUNTS>=2
-- 统计每个学生的选课门数，并按选课门数递减排序
SELECT S.NAME,COUNT(G.kecheng_id) FROM t_student S LEFT JOIN t_grade G ON S.ID=G.student_id GROUP BY S.ID ORDER BY COUNT(G.kecheng_id) DESC
--查询学生都选修了那些课程，要求列出课程名，课程号，选修人数以及最好成绩
select s.dept,g.kecheng_id,COUNT(g.kecheng_id)counts,g.grade from t_student s LEFT JOIN t_grade g on s.ID=g.student_id
GROUP BY g.kecheng_id
--查询分数大于90的学生姓名，姓名不能重复：
SELECT DISTINCT B.NAME FROM(
SELECT A.NAME FROM (
SELECT NAME,FENSHU,kecheng FROM t_student WHERE KECHENG IN ("语文","数学","英语"))A WHERE FENSHU >=90) B
--查询学生总分大于210的学生姓名和总分
SELECT A.NAME,A.SUMS FROM (
SELECT NAME,SUM(FENSHU) SUMS FROM T_STUDENT GROUP BY NAME )A WHERE SUMS>210
}