【MySQL】表的增删查改（进阶）

目录

1.数据库约束

1.1NOT NULL：非空约束

1.2UNIQUE：唯一值约束

1.3DEFAULT：默认值约束

1.4PRIMARY KEY：主键约束

1.5FOREIGN KEY：外键约束

1.6CHECK约束

2.表的设计

2.1一对一

2.2一对多

2.3多对多

3.新增

4.查询

4.1聚合查询

4.1.1聚合查询

4.1.2GROUP BY

4.2联合查询

4.2.1内连接 

4.2.2外连接

4.2.3自连接

4.2.4子查询

4.2.5合并查询

---

1.数据库约束

1.1NOT NULL：非空约束

mysql> create table student (id int, name varchar(20) not null);
Query OK, 0 rows affected (0.03 sec)

mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into student values (null,null);
ERROR 1048 (23000): Column 'name' cannot be null //id可以为空但是name不能为空

1.2UNIQUE：唯一值约束

不允许存在两行数据，在这个指定列上重复。

mysql> create table student(id int unique,name varchar(20));
Query OK, 0 rows affected (0.03 sec)

mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  | UNI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into student values(1,'zhangsan');
Query OK, 1 row affected (0.00 sec)

mysql> insert into student values(1,'wangwu');
ERROR 1062 (23000): Duplicate entry '1' for key 'id'//重复条目

1.3DEFAULT：默认值约束

可自定义默认值

mysql> create table student (id int,name varchar(20) default '未命名');
Query OK, 0 rows affected (0.05 sec)

mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | 未命名  |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into student (id) values (1);
Query OK, 1 row affected (0.04 sec)

mysql> select * from student;
+------+--------+
| id   | name   |
+------+--------+
|    1 | 未命名 |
+------+--------+
1 row in set (0.00 sec)

1.4PRIMARY KEY：主键约束

主键就是一条数据的身份标识，是not null 和 unique 的合体；

在数据库中，通过主键约束来指定某个列作为主键；

一个表中只能有一个主键

mysql> create table student (id int primary key,name varchar(20));
Query OK, 0 rows affected (0.05 sec)

mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into student values(1,'zhangsan');
Query OK, 1 row affected (0.00 sec)

mysql> insert into student values(null,'zhangsan');
ERROR 1048 (23000): Column 'id' cannot be null
mysql> insert into student values(1,'zhangsan');
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

自增主键：auto_increment，允许客户端在插入数据时，不手动指定主键的值，即可以是null，而是交给MySQL自行分配，确保分配出来的这个主键的值是和之前不重复的，也可以手动指定值。

mysql> create table student (id int primary key auto_increment,name varchar(20));
Query OK, 0 rows affected (0.05 sec)

mysql> insert into student values(null,'zhangsan');
Query OK, 1 row affected (0.04 sec)

mysql> insert into student values(null,'wangwu');
Query OK, 1 row affected (0.04 sec)

mysql> insert into student values(8,'zhaoliu');
Query OK, 1 row affected (0.04 sec)

mysql> insert into student values(null,'sunquan');
Query OK, 1 row affected (0.04 sec)

mysql> select * from student;
+----+----------+
| id | name     |
+----+----------+
|  1 | zhangsan |
|  2 | lisi     |
|  8 | zhaoliu  |
|  9 | sunquan  |
+----+----------+
4 rows in set (0.00 sec)

1.5FOREIGN KEY：外键约束

格式：foreign key（子表中受到约束的列）reference 父表 （父表中起到约束效果的列）;

创建外键时，修改的是子表的代码，父表代码不受影响。

mysql> create table class (classId int primary key,className varchar(20));
Query OK, 0 rows affected (0.02 sec)

mysql> insert into class values (1,'class602'),
    -> (2,'class709'),
    -> (3,'class303');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> create table student (studentId int primary key auto_increment, name varchar(20),classId int,
    -> foreign key(classId) references class(classId));
Query OK, 0 rows affected (0.03 sec)

注意：

1> 插入/修改子表中受约束的这一列的数据，要保证结果在父表中存在；

2> 删除/修改父表中的数据，要看看这个数据是否在子表中被使用了，如果被使用了，则不能进行删除/修改； 

3> 删表时要先删子表再删父表，否则删不掉

mysql> insert into student values(1,'zhangsan',8);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`javasql`.`student`, CONSTRAINT `student_ibfk_1` FOREIGN KEY (`classId`) REFERENCES `class` (`classId`))

mysql> insert into student values(1,'zhangsan',3);
Query OK, 1 row affected (0.01 sec)

mysql> delete from class where classId = 2;
Query OK, 1 row affected (0.01 sec)

mysql> delete from class where classId = 3;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`javasql`.`student`, CONSTRAINT `student_ibfk_1` FOREIGN KEY (`classId`) REFERENCES `class` (`classId`))

mysql> drop table class;
ERROR 1217 (23000): Cannot delete or update a parent row: a foreign key constraint fails
mysql> drop table student;
Query OK, 0 rows affected (0.02 sec)

mysql> drop table class;
Query OK, 0 rows affected (0.01 sec)

1.6CHECK约束

写一个具体的条件表达式，符合条件则可以插入/修改，不符合条件就失败；

对MySQL5来说，并不支持check，写上不会报错也没啥作用。

2.表的设计

关系是实体和实体之间的关联关系，数据库中有四种关系：

一对一，一对多，多对多，没关系

2.1一对一

举例：学生与账号

1> student ( id, name, age, classId……)

     account ( accountId, username, password, studentId……)

2> student ( id, name, age, classId, accountId……)

     account ( accountId, username, password……)

3> student_account ( id, name, classId, username, password……) 两张表放一起

2.2一对多

举例：学生与班级

class (classId, className……)

student (studentId, name, classId……)

2.3多对多

表示多对多关系时需要引入一个关联表

举例：学生与课程

student (id, name……)

course (id, name……)

student_course ( studentId, courseId) --- 关联表

3.新增

格式：insert into 表名 select ……

将 插入 insert 和 查询 select 操作合并在一起，即将查询出来的结果直接插入另一个表中，注意：两个表列的数目，类型，约束等要一一对应。

mysql> create table student (id int,name varchar(20));
Query OK, 0 rows affected (0.05 sec)

mysql> create table student2 (id int,name varchar(20));
Query OK, 0 rows affected (0.01 sec)

mysql> insert into student values (1,'zhangsan'),(2,'lisi'),(3,'wangwu');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> insert into student2 select * from student;
Query OK, 3 rows affected (0.04 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from student2;
+------+----------+
| id   | name     |
+------+----------+
|    1 | zhangsan |
|    2 | lisi     |
|    3 | wangwu   |
+------+----------+
3 rows in set (0.00 sec)

mysql> insert into student2 select name from student;
ERROR 1136 (21S01): Column count doesn't match value count at row 1
mysql> insert into student2 select name,id from student;
ERROR 1366 (HY000): Incorrect integer value: 'zhangsan' for column 'id' at row 1

4.查询

4.1聚合查询

4.1.1聚合查询

函数	说明
COUNT( [ DISTINCT ] expr)	返回查询到的数据的 数量
SUM( [ DISTINCT ] expr)	返回查询到的数据的 总和，不是数字没有意义
AVG( [ DISTINCT ] expr)	返回查询到的数据的 平均值，不是数字没有意义
MAX( [ DISTINCT ] expr)	返回查询到的数据的 最大值，不是数字没有意义
MIN( [ DISTINCT ] expr)	返回查询到的数据的 最小值，不是数字没有意义

COUNT：

计算数据的数量时，有时会不计入空值的个数；

空格不能乱敲，count  () 会报错，正确为count()。 

mysql> insert into examresult values (null,null,null,null,null);
Query OK, 1 row affected (0.00 sec)

mysql> select * from examresult;
+------+----------+---------+------+---------+
| id   | name     | Chinese | Math | English |
+------+----------+---------+------+---------+
|    1 | zhangsan |    67.0 | 98.0 |    56.0 |
|    2 | lisi     |    87.0 | 58.0 |    98.0 |
|    3 | wangwu   |    88.0 | 66.0 |    90.0 |
|    5 | 孙权     |    67.0 | 89.0 |    72.0 |
|    6 | 孙尚香   |    80.0 | 98.0 |    90.0 |
|    7 | 孙悟空   |    NULL | NULL |    NULL |
| NULL | NULL     |    NULL | NULL |    NULL |
+------+----------+---------+------+---------+
7 rows in set (0.00 sec)

mysql> select count(*) from examresult;
+----------+
| count(*) |
+----------+
|        7 |
+----------+
1 row in set (0.00 sec)

mysql> select count(name) from examresult;
+-------------+
| count(name) |
+-------------+
|           6 |
+-------------+
1 row in set (0.01 sec)

SUM：

将每一列的这一行先转化成double，再按照double的方式进行累加；

若无法相加，也能编译成功，但会报警告。

mysql> select sum(Chinese) from examresult;
+--------------+
| sum(Chinese) |
+--------------+
|        389.0 |
+--------------+
1 row in set (0.01 sec)

mysql> select sum(name) from examresult;
+-----------+
| sum(name) |
+-----------+
|         0 |
+-----------+
1 row in set, 6 warnings (0.00 sec)

mysql> show warnings;
+---------+------+----------------------------------------------+
| Level   | Code | Message                                      |
+---------+------+----------------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'zhangsan' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'lisi'     |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'wangwu'   |
| Warning | 1292 | Truncated incorrect DOUBLE value: '孙权'     |
| Warning | 1292 | Truncated incorrect DOUBLE value: '孙尚香'   |
| Warning | 1292 | Truncated incorrect DOUBLE value: '孙悟空'   |
+---------+------+----------------------------------------------+
6 rows in set (0.00 sec)

AVG、MAX、MIN：

mysql> select avg(Chinese + Math + English) from examresult;
+-------------------------------+
| avg(Chinese + Math + English) |
+-------------------------------+
|                     240.80000 |
+-------------------------------+
1 row in set (0.01 sec)

mysql> select max(Chinese) from examresult;
+--------------+
| max(Chinese) |
+--------------+
|         88.0 |
+--------------+
1 row in set (0.01 sec)

mysql> select min(English) from examresult;
+--------------+
| min(English) |
+--------------+
|         56.0 |
+--------------+
1 row in set (0.00 sec)

4.1.2GROUP BY

指定这一列，按照这一列进行分组，这一列中，数值相同的行会被放到同一组；

注意：查询时select中指定的列必须是当前group by 指定的列，如果想用其他的列，其他的列必须放到聚合函数中，否则查询的结果无意义。

mysql> select * from emp;
+----+--------+--------+--------+
| id | name   | role   | salary |
+----+--------+--------+--------+
|  1 | 张三   | 讲师   |  10000 |
|  2 | 李四   | 讲师   |  11000 |
|  3 | 王五   | 讲师   |  12000 |
|  4 | 赵六   | 学管师 |   9000 |
|  5 | 田七   | 学管师 |   8500 |
|  6 | 龙傲天 | 老板   | 100000 |
+----+--------+--------+--------+
6 rows in set (0.00 sec)

mysql> select role,avg(salary) from emp group by role;
+--------+-------------+
| role   | avg(salary) |
+--------+-------------+
| 学管师 |   8750.0000 |
| 老板   | 100000.0000 |
| 讲师   |  11000.0000 |
+--------+-------------+
3 rows in set (0.01 sec)

mysql> select role,name ,avg(salary) from emp group by role; //无意义
+--------+--------+-------------+
| role   | name   | avg(salary) |
+--------+--------+-------------+
| 学管师 | 赵六   |   8750.0000 |
| 老板   | 龙傲天 | 100000.0000 |
| 讲师   | 张三   |  11000.0000 |
+--------+--------+-------------+
3 rows in set (0.00 sec)

分组前的条件语句：where

分组后的条件语句：having

mysql> select role,avg(salary) from emp where name != '张三' group by role;
+--------+-------------+
| role   | avg(salary) |
+--------+-------------+
| 学管师 |   8750.0000 |
| 老板   | 100000.0000 |
| 讲师   |  11500.0000 |
+--------+-------------+
3 rows in set (0.01 sec)

mysql> select role,avg(salary) from emp group by role having avg(salary) < 20000;
+--------+-------------+
| role   | avg(salary) |
+--------+-------------+
| 学管师 |   8750.0000 |
| 讲师   |  11000.0000 |
+--------+-------------+
2 rows in set (0.01 sec)

mysql> select role,avg(salary) from emp where name != '张三' group by role having avg(salary) < 20000;
+--------+-------------+
| role   | avg(salary) |
+--------+-------------+
| 学管师 |   8750.0000 |
| 讲师   |  11500.0000 |
+--------+-------------+
2 rows in set (0.00 sec)

4.2联合查询

把所有表结合再筛选，效率十分低下。

笛卡尔积：任意两张表都可以算，但是如果两张表没有关系，计算的结果无意义。

       格式：select * from 表名, 表名……

连接条件：筛选掉无意义的数据。

建表阶段：

mysql> create table classes(id int primary key auto_increment, name varchar(20), `desc` varchar(100));
Query OK, 0 rows affected (0.02 sec)

mysql> create table student(id int primary key auto_increment, sn varchar(20), name varchar(20), qq_mail varchar(20), class_id int);
Query OK, 0 rows affected (0.02 sec)

mysql> create table course(id int primary key auto_increment, name varchar(20));
Query OK, 0 rows affected (0.02 sec)

mysql> create table score(score decimal(3,1), student_id int, course_id int);
Query OK, 0 rows affected (0.02 sec)

mysql> insert into classes(name, `desc`) values
    -> ('计算机系2022级16班', '计算机原理、C语言、Java语言'),
    -> ('中文系2022级3班','中国传统文学、古代文学鉴赏'),
    -> ('自动化2022级9班','机械自动化');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> insert into student(sn, name, qq_mail, class_id) values
    -> ('09982','张三','[email protected]',1),
    -> ('00835','李四',null,1),
    -> ('00391','王五',null,1),
    -> ('00031','赵六','[email protected]',1),
    -> ('00054','田七',null,1),
    -> ('51234','钱八','[email protected]',2),
    -> ('83223','孙九',null,2),
    -> ('09527','吴十','[email protected]',2);
Query OK, 8 rows affected (0.01 sec)
Records: 8  Duplicates: 0  Warnings: 0

mysql> insert into course(name) values
    -> ('Java'),('中国传统文化'),('计算机原理'),('语文'),('高阶数学'),('英文');
Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> insert into score(score, student_id, course_id) values
    -> (70.5, 1, 1),(98.5, 1, 3),(33, 1, 5),(98, 1, 6),
    -> (60, 2, 1),(59.5, 2, 5),
    -> (33, 3, 1),(68, 3, 3),(99, 3, 5),
    -> (67, 4, 1),(23, 4, 3),(56, 4, 5),(72, 4, 6),
    -> (81, 5, 1),(37, 5, 5),
    -> (56, 6, 2),(43, 6, 4),(79, 6, 6),
    -> (80, 7, 2),(92, 7, 6);
Query OK, 20 rows affected (0.01 sec)
Records: 20  Duplicates: 0  Warnings: 0

提问：查询赵六同学的成绩；

           查询每个班有多少人；

           查询每个同学的总成绩；

           查询每个同学每个课程的成绩

单表查询条件中直接写列名即可，多表查询条件中最好写作 “表名.列名” 

4.2.1内连接 

mysql> select student.name, score.score from student ,score where student.id = score.student_id and student.name = '赵六';
+------+-------+
| name | score |
+------+-------+
| 赵六 |  67.0 |
| 赵六 |  23.0 |
| 赵六 |  56.0 |
| 赵六 |  72.0 |
+------+-------+
4 rows in set (0.01 sec)

mysql> select classes.name, count(student.class_id) from student, classes where student.class_id = classes.id group by classes.name;
+--------------------+-------------------------+
| name               | count(student.class_id) |
+--------------------+-------------------------+
| 中文系2022级3班    |                       3 |
| 计算机系2022级16班 |                       5 |
+--------------------+-------------------------+
2 rows in set (0.00 sec)

mysql> select student.name, sum(score.score) as summary from student, score where student.id = score.student_id group by student.name;
+------+---------+
| name | summary |
+------+---------+
| 孙九 |   172.0 |
| 张三 |   300.0 |
| 李四 |   119.5 |
| 王五 |   200.0 |
| 田七 |   118.0 |
| 赵六 |   218.0 |
| 钱八 |   178.0 |
+------+---------+
7 rows in set (0.00 sec)

mysql> select student.name, course.name as object, score.score from student, score, course where student.id = score.student_id and score.course_id = course.id;
+------+--------------+-------+
| name | object       | score |
+------+--------------+-------+
| 张三 | Java         |  70.5 |
| 张三 | 计算机原理   |  98.5 |
| 张三 | 高阶数学     |  33.0 |
| 张三 | 英文         |  98.0 |
| 李四 | Java         |  60.0 |
| 李四 | 高阶数学     |  59.5 |
| 王五 | Java         |  33.0 |
| 王五 | 计算机原理   |  68.0 |
| 王五 | 高阶数学     |  99.0 |
| 赵六 | Java         |  67.0 |
| 赵六 | 计算机原理   |  23.0 |
| 赵六 | 高阶数学     |  56.0 |
| 赵六 | 英文         |  72.0 |
| 田七 | Java         |  81.0 |
| 田七 | 高阶数学     |  37.0 |
| 钱八 | 中国传统文化 |  56.0 |
| 钱八 | 语文         |  43.0 |
| 钱八 | 英文         |  79.0 |
| 孙九 | 中国传统文化 |  80.0 |
| 孙九 | 英文         |  92.0 |
+------+--------------+-------+
20 rows in set (0.00 sec)

4.2.2外连接

格式：select 列名 from 表名 left/right join 表名 on 条件；

分类：左外连接：left join 以左侧表为主，左侧表中的数据在结果中一定存在，若左表存在右表不存在，会将对应的列填为空值；

           右外连接：right join 类上；

           全外连接：outer join ,MySQL 不支持，Oracle支持。

mysql> select student.name, score.score from student join score on student.id = score.student_id and student.name = '赵 六';
+------+-------+
| name | score |
+------+-------+
| 赵六 |  67.0 |
| 赵六 |  23.0 |
| 赵六 |  56.0 |
| 赵六 |  72.0 |
+------+-------+
4 rows in set (0.00 sec)

mysql> select * from student;
+------+------+
| id   | name |
+------+------+
|    1 | 张三 |
|    2 | 李四 |
|    3 | 王五 |
+------+------+
3 rows in set (0.01 sec)

mysql> select * from score;
+------+-------+
| id   | score |
+------+-------+
|    1 |    90 |
|    2 |    80 |
|    4 |    70 |
+------+-------+
3 rows in set (0.00 sec)

mysql> select * from student join score on student.id = score.id;
+------+------+------+-------+
| id   | name | id   | score |
+------+------+------+-------+
|    1 | 张三 |    1 |    90 |
|    2 | 李四 |    2 |    80 |
+------+------+------+-------+
2 rows in set (0.00 sec)

mysql> select * from student left join score on student.id = score.id;
+------+------+------+-------+
| id   | name | id   | score |
+------+------+------+-------+
|    1 | 张三 |    1 |    90 |
|    2 | 李四 |    2 |    80 |
|    3 | 王五 | NULL |  NULL |
+------+------+------+-------+
3 rows in set (0.00 sec)

mysql> select * from student right join score on student.id = score.id;
+------+------+------+-------+
| id   | name | id   | score |
+------+------+------+-------+
|    1 | 张三 |    1 |    90 |
|    2 | 李四 |    2 |    80 |
| NULL | NULL |    4 |    70 |
+------+------+------+-------+
3 rows in set (0.00 sec)

4.2.3自连接

自己和自己做笛卡尔积，把行之间的关系转换为列之间的关系；

为避免重命名，名字要使用别名。

格式：select * from 表名 as 别名1, 表名 as 别名2;

mysql> select * from student1;
+------+------+
| id   | name |
+------+------+
|    1 | 张三 |
|    2 | 李四 |
|    3 | 王五 |
+------+------+
3 rows in set (0.00 sec)

mysql> select * from student1 as s1, student1 as s2;
+------+------+------+------+
| id   | name | id   | name |
+------+------+------+------+
|    1 | 张三 |    1 | 张三 |
|    2 | 李四 |    1 | 张三 |
|    3 | 王五 |    1 | 张三 |
|    1 | 张三 |    2 | 李四 |
|    2 | 李四 |    2 | 李四 |
|    3 | 王五 |    2 | 李四 |
|    1 | 张三 |    3 | 王五 |
|    2 | 李四 |    3 | 王五 |
|    3 | 王五 |    3 | 王五 |
+------+------+------+------+
9 rows in set (0.00 sec)

4.2.4子查询

嵌在其他SQL语句中的select语句，也叫嵌套查询。

4.2.5合并查询

将两个查询表合并成一个表。

格式：select * from 表1 union (all) select * from 表2;

合并前提：两表参数类型需一一对应，名字可以不同。

mysql> select * from student1;
+------+------+
| id   | name |
+------+------+
|    1 | 张三 |
|    2 | 李四 |
+------+------+
2 rows in set (0.04 sec)

mysql> select * from student2;
+------+------+
| id   | name |
+------+------+
|    1 | 张三 |
|    3 | 王五 |
+------+------+
2 rows in set (0.00 sec)

mysql> select * from student1 union select * from student2;
+------+------+
| id   | name |
+------+------+
|    1 | 张三 |
|    2 | 李四 |
|    3 | 王五 |
+------+------+
3 rows in set (0.04 sec)

mysql> select * from student1 union all select * from student2;
+------+------+
| id   | name |
+------+------+
|    1 | 张三 |
|    2 | 李四 |
|    1 | 张三 |
|    3 | 王五 |
+------+------+
4 rows in set (0.01 sec)

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关于表的内容就over啦！