MySQL 数据查询小练习

作业

# 创建班级表
create table class (
cls_id int auto_increment primary key,
cls_name varchar(10) not null default ''
);

insert into class (cls_name) values ('三年二班'), ('一年三班'), ('二年一班');

# 创建老师表
create table teacher (
t_id int auto_increment primary key,
t_name varchar(10) not null default ''
);

insert into teacher (t_name) values ('罗老师'), ('厨子'), ('乔帮主');

#创建学生表
create table student (
s_id int auto_increment primary key,
s_name varchar(10) not null default '',
gender enum('男', '女') not null default '男',
class_id int not null default 1,
constraint fk_std_cls foreign key (class_id) references class(cls_id)
);

insert into student (s_name, gender, class_id ) values ('雷军', '男', 1), ('余大嘴', '男', 2), ('黄章', '女', 3);

# 创建课程表
create table course (
c_id int auto_increment primary key,
c_name varchar(10) not null default '',
teacher_id int not null default 1,
constraint fk_course_teacher foreign key (teacher_id ) references teacher(t_id)
);

insert into course (c_name, teacher_id ) values ('Python', 3), ('C', 1), ('Go',2);

# 创建成绩表
create table score (
scr_id int auto_increment primary key,
student_id int not null default 1,
course_id int not null default 1,
scr_num int not null default 0,
constraint fk_scr_std foreign key (student_id ) references student(s_id),
constraint fk_scr_crs foreign key (course_id ) references course(c_id)
);

insert into score (student_id, course_id, scr_num) values (1, 3, 80), (2, 1, 70), (1, 2, 60);

# 班级
+--------+----------+
| cls_id | cls_name |
+--------+----------+
|      1 | 三年二班 |
|      2 | 一年三班 |
|      3 | 二年一班 |
+--------+----------+

# 老师
+------+--------+
| t_id | t_name |
+------+--------+
|    1 | 罗老师 |
|    2 | 厨子   |
|    3 | 乔帮主 |
+------+--------+

# 学生
+------+--------+--------+----------+
| s_id | s_name | gender | class_id |
+------+--------+--------+----------+
|    1 | 雷军   | 男     |        1 |
|    2 | 余大嘴 | 男     |        2 |
|    3 | 黄章   | 女     |        3 |
+------+--------+--------+----------+

# 课程
+------+--------+------------+
| c_id | c_name | teacher_id |
+------+--------+------------+
|    1 | Python |          3 |
|    2 | C      |          1 |
|    3 | Go     |          2 |
+------+--------+------------+

# 分数
+--------+------------+-----------+---------+
| scr_id | student_id | course_id | scr_num |
+--------+------------+-----------+---------+
|      1 |          1 |         3 |      80 |
|      2 |          2 |         1 |      70 |
|      3 |          1 |         2 |      60 |
+--------+------------+-----------+---------+

• 查询所有大于60分的学生的姓名和学号 (DISTINCT: 去重)

mysql> select distinct s_name, student_id  from score left join student on student_id=student.s_id where scr_num > 60;
+--------+------------+
| s_name | student_id |
+--------+------------+
| 雷军   |          1 |
| 余大嘴 |          2 |
+--------+------------+
2 rows in set (0.00 sec)

• 查询每个老师教授的课程数量和老师信息

mysql> select t_name, count(t_name) as crs_count from course left join teacher on teacher_id=teacher.t_id group by t_name;
+--------+-----------+
| t_name | crs_count |
+--------+-----------+
| 乔帮主 |         1 |
| 厨子   |         1 |
| 罗老师 |         1 |
+--------+-----------+
3 rows in set (0.00 sec)

• 查询学生的信息以及学生所在的班级信息

mysql> select * from student left join class on class_id=class.cls_id;
+------+--------+--------+----------+--------+----------+
| s_id | s_name | gender | class_id | cls_id | cls_name |
+------+--------+--------+----------+--------+----------+
|    1 | 雷军   | 男     |        1 |      1 | 三年二班 |
|    2 | 余大嘴 | 男     |        2 |      2 | 一年三班 |
|    3 | 黄章   | 女     |        3 |      3 | 二年一班 |
+------+--------+--------+----------+--------+----------+
3 rows in set (0.00 sec)

• 学生中男生的个数和女生的个数

mysql> select gender, count(gender) from student group by gender;
+--------+---------------+
| gender | count(gender) |
+--------+---------------+
| 男     |             2 |
| 女     |             1 |
+--------+---------------+
2 rows in set (0.00 sec)

• 获取所有学习 "Python" 的学生的学号, 成绩和姓名

mysql> select c_name, s_id, s_name, scr_num from score left join student on student_id=student.s_id left join course on course_id=course.c_id where c_name='Python';
+--------+------+--------+---------+
| c_name | s_id | s_name | scr_num |
+--------+------+--------+---------+
| Python |    2 | 余大嘴 |      70 |
+--------+------+--------+---------+
1 row in set (0.00 sec)

• 查询平均成绩大于60分的同学的信息和平均成绩

mysql> select s_id, s_name, avg(scr_num) from score left join student on student_id=student.s_id group by s_name having avg(scr_num) > 60;
+------+--------+--------------+
| s_id | s_name | avg(scr_num) |
+------+--------+--------------+
|    2 | 余大嘴 |      70.0000 |
|    1 | 雷军   |      70.0000 |
+------+--------+--------------+
2 rows in set (0.00 sec)

• 查询姓“罗”的老师的个数；

mysql> select t_name, count(t_name) from teacher group by t_name having t_name like '罗%';
+--------+---------------+
| t_name | count(t_name) |
+--------+---------------+
| 罗老师 |             1 |
+--------+---------------+

• 查询课程成绩小于70分的同学的学号, 姓名

mysql> select s_id, s_name, scr_num from score left join student on student_id=student.s_id where scr_num < 70;
+------+--------+---------+
| s_id | s_name | scr_num |
+------+--------+---------+
|    1 | 雷军   |      60 |
+------+--------+---------+
1 row in set (0.00 sec)

• 查询各科成绩最高和最低的分：以如下形式显示：课程名，最高分，最低分；

mysql> select c_name, max(scr_num), min(scr_num) from score left join course on course_id=course.c_id group by c_name;
+--------+--------------+--------------+
| c_name | max(scr_num) | min(scr_num) |
+--------+--------------+--------------+
| C      |           60 |           60 |
| Go     |           80 |           80 |
| Python |           70 |           70 |
+--------+--------------+--------------+
3 rows in set (0.00 sec)

• 查询姓“雷”的学生名单

mysql> select * from student where s_name like '雷%';
+------+--------+--------+----------+
| s_id | s_name | gender | class_id |
+------+--------+--------+----------+
|    1 | 雷军   | 男     |        1 |
+------+--------+--------+----------+
1 row in set (0.00 sec)

• 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

mysql> select c_name, avg(scr_num) from score left join course on course_id=course.c_id group by c_name order by avg(scr_num) asc, c_id desc;
+--------+--------------+
| c_name | avg(scr_num) |
+--------+--------------+
| C      |      60.0000 |
| Python |      70.0000 |
| Go     |      80.0000 |
+--------+--------------+
3 rows in set (0.00 sec)

• 查询课程编号为3且课程成绩在70分以上的学生的学号和姓名

mysql> select s_id, s_name from score left join student on student_id=student.s_id left join course on course_id=course.c_id where c_id=3 and scr_num > 70;
+------+--------+
| s_id | s_name |
+------+--------+
|    1 | 雷军   |
+------+--------+
1 row in set (0.00 sec)

• 查询“2”课程分数小于70，按分数降序排列的同学学号

mysql> select s_id from score left join student on student_id=student.s_id left join course on course_id=course.c_id where c_id=2 and scr_num < 70 order by scr_num desc;
+------+
| s_id |
+------+
|    1 |
+------+
1 row in set (0.00 sec)

• 删除学号为“2”的同学的“1”课程的成绩

mysql> delete from score where student_id=2 and course_id=1;
Query OK, 1 row affected (0.00 sec)

mysql> select * from score;
+--------+------------+-----------+---------+
| scr_id | student_id | course_id | scr_num |
+--------+------------+-----------+---------+
|      1 |          1 |         3 |      80 |
|      3 |          1 |         2 |      60 |
+--------+------------+-----------+---------+
2 rows in set (0.00 sec)

• 删除学习“乔帮主”课的SC表记录