数据库学习-表结构、键值

回顾数据类型
表结构
- 字段约束条件（限制字段赋值的）
- - 例，建表并设置约束条件
  - 设置default列为0
- 修改表结构
- - 例添加新字段，并设置位置。
  - 修改字段类型
  - change 修改字段名
  - 注意：在修改字段名或字段类型时，如果此时相应列已有数据，会报错。！！！要修改的内容，不能与已有的值冲突！！！
  - drop 删字段（列）
  - 修改表名
MySQL的键值,key（标签）
- 索引类型（排列方式）
- 普通索引 ( 索引：类似书的目录，让列的值排队)
- - 在创建表的同时，添加索引
  - 查索引情况
  - 删索引
- 主键
- - 建表时，定义主键的方式
  - 修改、删除主键。！！注：取消主键后的列，‘null’列为NO，YES 要手动改！！
  - 如果字段下面的值，有相同的，则无法创建主键！！！
- 复合主键（表里的多列一起定义为主键，复合主键的值不允许同时一样！！）
- - 修改、删除主键
- 主键与auto_increment（自动增长+1）连用。需要字段为主键，列的类型为数值类型！！！
- - 按唯一编号，去查找信息
  - 有自增长的主键，要先删掉自增长才能删主键。2、按上述表状态就算把表情空，再添加记录，编号也会按清空前的最后一位+1。3、要想重新排序，删字段再重建。
- 外键
- - 建员工和工资表，指定外键工资列的id与员工表的对应，同步。
  - 验证是否有外键：
  - 添加不在条件范围内的，会报错！！
  - 验证，同步删除的效果
  - 修改，同步效果
  - 删外键：
  - 扩展，外键可存空值，这于表的设计思路不符，需要进行控制。把目标外键设置为主键，可避免为空和重复。

回顾数据类型

mysql> create table test9 ( name char(10), age tinyint(3) unsigned, address varchar(50), pay float(7,2), start year, birthday date,likes set('eat','game','film','music'), sex enum('boy','girl'));
Query OK, 0 rows affected, 2 warnings (0.46 sec)

mysql> desc test9;
+----------+----------------------------------+------+-----+---------+-------+
| Field    | Type                             | Null | Key | Default | Extra |
+----------+----------------------------------+------+-----+---------+-------+
| name     | char(10)                         | YES  |     | NULL    |       |
| age      | tinyint(3) unsigned              | YES  |     | NULL    |       |
| address  | varchar(50)                      | YES  |     | NULL    |       |
| pay      | float(7,2)                       | YES  |     | NULL    |       |
| start    | year(4)                          | YES  |     | NULL    |       |
| birthday | date                             | YES  |     | NULL    |       |
| likes    | set('eat','game','film','music') | YES  |     | NULL    |       |
| sex      | enum('boy','girl')               | YES  |     | NULL    |       |
+----------+----------------------------------+------+-----+---------+-------+
8 rows in set (0.01 sec)



mysql> insert into test9 values('lisi',16,'shenzhen-baoan-xixiang',782.53,1997,date(now()),'game,film','boy');
Query OK, 1 row affected (0.06 sec)

mysql> select * from test9;
+------+------+------------------------+--------+-------+------------+-----------+------+
| name | age  | address                | pay    | start | birthday   | likes     | sex  |
+------+------+------------------------+--------+-------+------------+-----------+------+
| lisi |   16 | shenzhen-baoan-xixiang | 782.53 |  1997 | 2023-05-06 | game,film | boy  |
+------+------+------------------------+--------+-------+------------+-----------+------+
1 row in set (0.03 sec)

表结构

字段约束条件（限制字段赋值的）

请添加图片描述

NULL：是否允许赋空值，默认允许，可设置
列名类型 not null;
.
key: 键值
.
default : 默认值，向表里添加新行时，不给列赋值，会使用默认值赋值
列名类型 default 值;
.

mysql> desc test11;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| money | float(8,3) | YES  |     | NULL    |       |
+-------+------------+------+-----+---------+-------+
1 row in set (0.01 sec)

mysql>  desc mysql.db;
+-----------------------+---------------+------+-----+---------+-------+
| Field                 | Type          | Null | Key | Default | Extra |
+-----------------------+---------------+------+-----+---------+-------+
| Host                  | char(255)     | NO   | PRI |         |       |
| Db                    | char(64)      | NO   | PRI |         |       |
| User                  | char(32)      | NO   | PRI |         |       |
| Select_priv           | enum('N','Y') | NO   |     | N       |       |
| Insert_priv           | enum('N','Y') | NO   |     | N       |       |
| Update_priv           | enum('N','Y') | NO   |     | N       |       |
| Delete_priv           | enum('N','Y') | NO   |     | N       |       |
| Create_priv           | enum('N','Y') | NO   |     | N       |       |
| Drop_priv             | enum('N','Y') | NO   |     | N       |       |

例，建表并设置约束条件

mysql> create table test10 (name char(10) not null, age tinyint(3) unsigned default "27", class char(10) not null default "class2021", up_time time );
Query OK, 0 rows affected, 1 warning (0.21 sec)

mysql> desc test10;
+---------+---------------------+------+-----+-----------+-------+
| Field   | Type                | Null | Key | Default   | Extra |
+---------+---------------------+------+-----+-----------+-------+
| name    | char(10)            | NO   |     | NULL      |       |
| age     | tinyint(3) unsigned | YES  |     | 27        |       |
| class   | char(10)            | NO   |     | class2021 |       |
| up_time | time                | YES  |     | NULL      |       |
+---------+---------------------+------+-----+-----------+-------+
4 rows in set (0.01 sec)



加表记录
mysql> insert into test10(name,up_time) values("bob",curtime());
Query OK, 1 row affected (0.04 sec)

mysql> insert into test10(name,up_time) values("bob",time(152325));
Query OK, 1 row affected (0.10 sec)


mysql> select * from test10;
+------+------+-----------+----------+
| name | age  | class     | up_time  |
+------+------+-----------+----------+
| bob  |   27 | class2021 | 20:52:27 |
| bob  |   27 | class2021 | 15:23:25 |
+------+------+-----------+----------+
2 rows in set (0.00 sec)



mysql> insert into test10 values("tom",null,"class2105",122345);

mysql> insert into test10 values("tom",22,"null",122345);



注：以下这两种情况，都不为空：
mysql> insert into test10(name) values("");  
#这里直接双引号，为0个字符，不是空
mysql> insert into test10(name) values("null");

mysql> insert into test10(name) values(null);
ERROR 1048 (23000): Column 'name' cannot be null


mysql> select * from test10;
+------+------+-----------+----------+
| name | age  | class     | up_time  |
+------+------+-----------+----------+
| bob  |   27 | class2021 | 20:52:27 |
| bob  |   27 | class2021 | 15:23:25 |
| tom  | NULL | class2105 | 12:23:45 |
| tom  |   22 | null      | 12:23:45 |
|      |   27 | class2021 | NULL     |
| null |   27 | class2021 | NULL     |
+------+------+-----------+----------+
6 rows in set (0.00 sec)

设置default列为0

mysql> create table test11 (
    -> name char(5) not null default "",
    -> age tinyint(3) default 25
    -> );
Query OK, 0 rows affected, 1 warning (0.13 sec)

mysql> desc test11;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| name  | char(5)    | NO   |     |         |       |
| age   | tinyint(3) | YES  |     | 25      |       |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.01 sec)


mysql> insert into test11 values ('bob',null);
Query OK, 1 row affected (0.00 sec)
mysql> insert into test11(name) values ('bob');

mysql> select * from test11;
+------+------+
| name | age  |
+------+------+
| bob  | NULL |
| bob  |   25 |
+------+------+
2 rows in set (0.00 sec)

修改表结构

请添加图片描述

例添加新字段，并设置位置。

不定义位置，默认在行尾。
定义到开头，加 first
定义到xx行后，为 after xx

mysql> alter table test11 add address char(20);
Query OK, 0 rows affected (0.45 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> select * from test11;
+--------+------+---------+
| name   | age  | address |
+--------+------+---------+
| bob    | NULL | NULL    |
| bob    |   25 | NULL    |
| 小明   |   25 | NULL    |
+--------+------+---------+
3 rows in set (0.00 sec)


mysql> alter table test11 add hobby char(20) after age, add sex enum('boy','girl') first;
# 不定义位置，默认在行尾。
# 定义到开头，加 first
# 定义到xx行后，为 after  xx  
Query OK, 0 rows affected (0.70 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc test11;
+---------+--------------------+------+-----+---------+-------+
| Field   | Type               | Null | Key | Default | Extra |
+---------+--------------------+------+-----+---------+-------+
| sex     | enum('boy','girl') | YES  |     | NULL    |       |
| name    | char(5)            | NO   |     |         |       |
| age     | tinyint(3)         | YES  |     | 25      |       |
| hobby   | char(20)           | YES  |     | NULL    |       |
| address | char(20)           | YES  |     | NULL    |       |
+---------+--------------------+------+-----+---------+-------+
5 rows in set (0.01 sec)

修改字段类型

请添加图片描述

mysql> alter table test11 modify hobby set('music','film','sleep','write');
Query OK, 3 rows affected (0.48 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> desc test11;
+---------+-------------------------------------+------+-----+---------+-------+
| Field   | Type                                | Null | Key | Default | Extra |
+---------+-------------------------------------+------+-----+---------+-------+
| sex     | enum('boy','girl')                  | YES  |     | NULL    |       |
| name    | char(5)                             | NO   |     |         |       |
| age     | tinyint(3)                          | YES  |     | 25      |       |
| hobby   | set('music','film','sleep','write') | YES  |     | NULL    |       |
| address | char(20)                            | YES  |     | NULL    |       |
+---------+-------------------------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

change 修改字段名

请添加图片描述

mysql> alter table test11 change  hobby likes set('music','film','sleep','write');
Query OK, 0 rows affected (0.33 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc test11;
+---------+-------------------------------------+------+-----+---------+-------+
| Field   | Type                                | Null | Key | Default | Extra |
+---------+-------------------------------------+------+-----+---------+-------+
| sex     | enum('boy','girl')                  | YES  |     | NULL    |       |
| name    | char(5)                             | NO   |     |         |       |
| age     | tinyint(3)                          | YES  |     | 25      |       |
| likes   | set('music','film','sleep','write') | YES  |     | NULL    |       |
| address | char(20)                            | YES  |     | NULL    |       |
+---------+-------------------------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

注意：在修改字段名或字段类型时，如果此时相应列已有数据，会报错。！！！要修改的内容，不能与已有的值冲突！！！

mysql> insert into test11 values ('girl','lili',36,'music','shenzhen');
Query OK, 1 row affected (0.09 sec)

mysql> select * from test11;
+------+--------+------+-------+----------+
| sex  | name   | age  | likes | address  |
+------+--------+------+-------+----------+
| NULL | bob    | NULL | NULL  | NULL     |
| NULL | bob    |   25 | NULL  | NULL     |
| NULL | 小明   |   25 | NULL  | NULL     |
| girl | lili   |   36 | music | shenzhen |
+------+--------+------+-------+----------+
4 rows in set (0.00 sec)

mysql> desc test11;
+---------+----------------------------------------+------+-----+---------+-------+
| Field   | Type                                   | Null | Key | Default | Extra |
+---------+----------------------------------------+------+-----+---------+-------+
| sex     | enum('boy','girl')                     | YES  |     | NULL    |       |
| name    | char(5)                                | NO   |     |         |       |
| age     | tinyint(3)                             | YES  |     | 25      |       |
| likes   | set('music','film','sleep','write')    | YES  |     | NULL    |       |
| address | enum('shenzhen','guangzhou','beijing') | YES  |     | NULL    |       |
+---------+----------------------------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)


# 这里要改喜好字段，因为喜欢列已经有了music的值，所以更改的内容里，不能与其冲突。再改不能把music去掉。
mysql> alter table test11 modify likes set('write','read','game');
ERROR 1265 (01000): Data truncated for column 'likes' at row 4

mysql> alter table test11 modify likes set('write','read','game','music');
Query OK, 4 rows affected (0.27 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> desc test11;
+---------+----------------------------------------+------+-----+---------+-------+
| Field   | Type                                   | Null | Key | Default | Extra |
+---------+----------------------------------------+------+-----+---------+-------+
| sex     | enum('boy','girl')                     | YES  |     | NULL    |       |
| name    | char(5)                                | NO   |     |         |       |
| age     | tinyint(3)                             | YES  |     | 25      |       |
| likes   | set('write','read','game','music')     | YES  |     | NULL    |       |
| address | enum('shenzhen','guangzhou','beijing') | YES  |     | NULL    |       |
+---------+----------------------------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

drop 删字段（列）

请添加图片描述

mysql> desc test11;
+---------+----------------------------------------+------+-----+---------+-------+
| Field   | Type                                   | Null | Key | Default | Extra |
+---------+----------------------------------------+------+-----+---------+-------+
| sex     | enum('boy','girl')                     | YES  |     | NULL    |       |
| name    | char(5)                                | NO   |     |         |       |
| age     | tinyint(3)                             | YES  |     | 25      |       |
| likes   | set('write','read','game','music')     | YES  |     | NULL    |       |
| address | enum('shenzhen','guangzhou','beijing') | YES  |     | NULL    |       |
+---------+----------------------------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

mysql> alter table test11 drop sex;
Query OK, 0 rows affected (0.30 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc test11;
+---------+----------------------------------------+------+-----+---------+-------+
| Field   | Type                                   | Null | Key | Default | Extra |
+---------+----------------------------------------+------+-----+---------+-------+
| name    | char(5)                                | NO   |     |         |       |
| age     | tinyint(3)                             | YES  |     | 25      |       |
| likes   | set('write','read','game','music')     | YES  |     | NULL    |       |
| address | enum('shenzhen','guangzhou','beijing') | YES  |     | NULL    |       |
+---------+----------------------------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

修改表名

mysql>  alter table test11 rename about_key;
Query OK, 0 rows affected (0.14 sec)

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| about_key     |
| st1           |
| st2           |

MySQL的键值,key（标签）

普通索引（index）
主键（primary key）
外键(foreign key)
唯一索引（unique）
全文索引（full text）
.
查看标签：desc 库.表
查看标签详细信息： show index from 库.表

索引类型（排列方式）

二叉树：btree
B+tree
hash

请添加图片描述

普通索引 ( 索引：类似书的目录，让列的值排队)

索引：
优：加快查找速度
缺：占用硬盘空间，写数据的速度会慢

请添加图片描述

mysql> create index haha on test11(name);
Query OK, 0 rows affected (0.29 sec)
Records: 0  Duplicates: 0  Warnings: 0


mysql> desc test11;
+---------+----------------------------------------+------+-----+---------+-------+
| Field   | Type                                   | Null | Key | Default | Extra |
+---------+----------------------------------------+------+-----+---------+-------+
| name    | char(5)                                | NO   | MUL |         |       |
| age     | tinyint(3)                             | YES  |     | 25      |       |
| likes   | set('write','read','game','music')     | YES  |     | NULL    |       |
| address | enum('shenzhen','guangzhou','beijing') | YES  |     | NULL    |       |
+---------+----------------------------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

在创建表的同时，添加索引

mysql> create table test12 (name char(5),id char(20), index(name));
Query OK, 0 rows affected (0.21 sec)

mysql> desc test12;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name  | char(5)  | YES  | MUL | NULL    |       |
| id    | char(20) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)

查索引情况

mysql> create index hehe on test11(age);
Query OK, 0 rows affected (0.19 sec)
Records: 0  Duplicates: 0  Warnings: 0


mysql> show index from test11 \G;
*************************** 1. row ***************************
        Table: test11
   Non_unique: 1
     Key_name: haha
 Seq_in_index: 1
  Column_name: name
    Collation: A
  Cardinality: 3
     Sub_part: NULL
       Packed: NULL
         Null:
   Index_type: BTREE
      Comment:
Index_comment:
      Visible: YES
   Expression: NULL
*************************** 2. row ***************************
        Table: test11
   Non_unique: 1
     Key_name: hehe
 Seq_in_index: 1
  Column_name: age
    Collation: A
  Cardinality: 3
     Sub_part: NULL
       Packed: NULL
         Null: YES
   Index_type: BTREE
      Comment:
Index_comment:
      Visible: YES
   Expression: NULL
2 rows in set (0.01 sec)

删索引

mysql> drop index hehe on test11;
Query OK, 0 rows affected (0.10 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from test11 \G;
*************************** 1. row ***************************
        Table: test11
   Non_unique: 1
     Key_name: haha
 Seq_in_index: 1
  Column_name: name
    Collation: A
  Cardinality: 3
     Sub_part: NULL
       Packed: NULL
         Null:
   Index_type: BTREE
      Comment:
Index_comment:
      Visible: YES
   Expression: NULL
1 row in set (0.00 sec)

ERROR:
No query specified

mysql> desc test11;
+---------+----------------------------------------+------+-----+---------+-------+
| Field   | Type                                   | Null | Key | Default | Extra |
+---------+----------------------------------------+------+-----+---------+-------+
| name    | char(5)                                | NO   | MUL |         |       |
| age     | tinyint(3)                             | YES  |     | 25      |       |
| likes   | set('write','read','game','music')     | YES  |     | NULL    |       |
| address | enum('shenzhen','guangzhou','beijing') | YES  |     | NULL    |       |
+---------+----------------------------------------+------+-----+---------+-------+
4 rows in set (0.01 sec)

主键

请添加图片描述

建表时，定义主键的方式

1、选项创建
mysql> create table test013(
    -> name char(10) primary key,
    -> age int(3)
    -> );
Query OK, 0 rows affected, 1 warning (0.10 sec)

mysql> desc test013;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name  | char(10) | NO   | PRI | NULL    |       |
| age   | int(3)   | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)


2、命令创建
mysql> create table test014 (
    -> name char(5),
    -> age int(3),
    -> sex enum('girl','boy'),
    -> primary key(name)
    -> );
Query OK, 0 rows affected, 1 warning (0.33 sec)

mysql> desc test014;
+-------+--------------------+------+-----+---------+-------+
| Field | Type               | Null | Key | Default | Extra |
+-------+--------------------+------+-----+---------+-------+
| name  | char(5)            | NO   | PRI | NULL    |       |
| age   | int(3)             | YES  |     | NULL    |       |
| sex   | enum('girl','boy') | YES  |     | NULL    |       |
+-------+--------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

修改、删除主键。！！注：取消主键后的列，‘null’列为NO，YES 要手动改！！

请添加图片描述

mysql> alter table test013 drop primary key;
Query OK, 0 rows affected (0.33 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc test013;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name  | char(10) | NO   |     | NULL    |       |
| age   | int(3)   | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)


mysql> alter table test013 modify name char(10);
Query OK, 0 rows affected (0.32 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc test013;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name  | char(10) | YES  |     | NULL    |       |
| age   | int(3)   | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)



2、修改主键：
mysql> alter table test013 add primary key(age);
Query OK, 0 rows affected (0.20 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc test013;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name  | char(10) | YES  |     | NULL    |       |
| age   | int(3)   | NO   | PRI | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)

如果字段下面的值，有相同的，则无法创建主键！！！

mysql> select * from test010;
+------+------+-----------+----------+
| name | age  | class     | up_time  |
+------+------+-----------+----------+
| bob  |   27 | class2021 | 20:52:27 |
| bob  |   27 | class2021 | 15:23:25 |
| tom  | NULL | class2105 | 12:23:45 |
| tom  |   22 | null      | 12:23:45 |
|      |   27 | class2021 | NULL     |
| null |   27 | class2021 | NULL     |
+------+------+-----------+----------+
6 rows in set (0.03 sec)

mysql> desc test010;
+---------+---------------------+------+-----+-----------+-------+
| Field   | Type                | Null | Key | Default   | Extra |
+---------+---------------------+------+-----+-----------+-------+
| name    | char(10)            | NO   |     | NULL      |       |
| age     | tinyint(3) unsigned | YES  |     | 27        |       |
| class   | char(10)            | NO   |     | class2021 |       |
| up_time | time                | YES  |     | NULL      |       |
+---------+---------------------+------+-----+-----------+-------+
4 rows in set (0.00 sec)

mysql> alter table test010 add primary key(name);
ERROR 1062 (23000): Duplicate entry 'bob' for key 'PRIMARY'
mysql> alter table test010 add primary key(age);
ERROR 1138 (22004): Invalid use of NULL value

复合主键（表里的多列一起定义为主键，复合主键的值不允许同时一样！！）

mysql> create table test015 (ip varchar(15), port smallint, status enum('open','no'), primary key(ip,port) );
Query OK, 0 rows affected (0.15 sec)

mysql> desc test015;
+--------+-------------------+------+-----+---------+-------+
| Field  | Type              | Null | Key | Default | Extra |
+--------+-------------------+------+-----+---------+-------+
| ip     | varchar(15)       | NO   | PRI | NULL    |       |
| port   | smallint(6)       | NO   | PRI | NULL    |       |
| status | enum('open','no') | YES  |     | NULL    |       |
+--------+-------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)


mysql> insert into test015 values ('192.168.12.12',22,'no');
ERROR 1062 (23000): Duplicate entry '192.168.12.12-22' for key 'PRIMARY'
mysql> insert into test015 values ('192.168.12.12',43,'no');
Query OK, 1 row affected (0.06 sec)

mysql> insert into test015 values ('192.168.12.12',23,'open');
Query OK, 1 row affected (0.01 sec)

mysql> insert into test015 values ('192.168.12.12',80,'no');
Query OK, 1 row affected (0.01 sec)


mysql> select * from test015;
+---------------+------+--------+
| ip            | port | status |
+---------------+------+--------+
| 192.168.12.12 |   22 | open   |
| 192.168.12.12 |   23 | open   |
| 192.168.12.12 |   43 | no     |
| 192.168.12.12 |   80 | no     |
+---------------+------+--------+
4 rows in set (0.00 sec)

修改、删除主键

mysql> alter table test015 drop primary key;
Query OK, 4 rows affected (0.21 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> desc test015;
+--------+-------------------+------+-----+---------+-------+
| Field  | Type              | Null | Key | Default | Extra |
+--------+-------------------+------+-----+---------+-------+
| ip     | varchar(15)       | NO   |     | NULL    |       |
| port   | smallint(6)       | NO   |     | NULL    |       |
| status | enum('open','no') | YES  |     | NULL    |       |
+--------+-------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> alter table test015 add primary key(ip,port);
Query OK, 0 rows affected (0.27 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc test015;
+--------+-------------------+------+-----+---------+-------+
| Field  | Type              | Null | Key | Default | Extra |
+--------+-------------------+------+-----+---------+-------+
| ip     | varchar(15)       | NO   | PRI | NULL    |       |
| port   | smallint(6)       | NO   | PRI | NULL    |       |
| status | enum('open','no') | YES  |     | NULL    |       |
+--------+-------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

主键与auto_increment（自动增长+1）连用。需要字段为主键，列的类型为数值类型！！！

mysql> create table test016 (
    -> id int primary key  auto_increment,
    -> name char(5),age tinyint(3),class char(10));
Query OK, 0 rows affected, 1 warning (0.16 sec)

mysql> desc test016;
+-------+------------+------+-----+---------+----------------+
| Field | Type       | Null | Key | Default | Extra          |
+-------+------------+------+-----+---------+----------------+
| id    | int(11)    | NO   | PRI | NULL    | auto_increment |
| name  | char(5)    | YES  |     | NULL    |                |
| age   | tinyint(3) | YES  |     | NULL    |                |
| class | char(10)   | YES  |     | NULL    |                |
+-------+------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)



mysql> insert into test016(name,age,class) values 

mysql> insert into test016(name,age,class) values 

mysql> insert into test016(name,age,class) values )

mysql> select * from test016;
+----+------+------+-----------+
| id | name | age  | class     |
+----+------+------+-----------+
|  1 | bob  |   26 | class2305 |
|  2 | tom  |   26 | class2305 |
|  3 | lisi |   35 | class2305 |
+----+------+------+-----------+
3 rows in set (0.00 sec)



2、自定义编号值
mysql> insert into test016  values (5,'haha',35,'class2305');
Query OK, 1 row affected (0.06 sec)

mysql> insert into test016  values (null,'haha',35,'class2305');
Query OK, 1 row affected (0.02 sec)
# 赋值空，会继续按照自+1进行

mysql> select * from test016;
+----+------+------+-----------+
| id | name | age  | class     |
+----+------+------+-----------+
|  1 | bob  |   26 | class2305 |
|  2 | tom  |   26 | class2305 |
|  3 | lisi |   35 | class2305 |
|  5 | haha |   35 | class2305 |
|  6 | haha |   35 | class2305 |
+----+------+------+-----------+
5 rows in set (0.01 sec)

按唯一编号，去查找信息

mysql> select * from test016 where id=3;
+----+------+------+-----------+
| id | name | age  | class     |
+----+------+------+-----------+
|  3 | lisi |   35 | class2305 |
+----+------+------+-----------+
1 row in set (0.01 sec)


mysql> select * from test016 where id>=3;
+----+------+------+-----------+
| id | name | age  | class     |
+----+------+------+-----------+
|  3 | lisi |   35 | class2305 |
|  5 | haha |   35 | class2305 |
|  6 | haha |   35 | class2305 |
+----+------+------+-----------+
3 rows in set (0.06 sec)

有自增长的主键，要先删掉自增长才能删主键。2、按上述表状态就算把表情空，再添加记录，编号也会按清空前的最后一位+1。3、要想重新排序，删字段再重建。

mysql> delete from test016;
Query OK, 5 rows affected (0.07 sec)

mysql> insert into test016  values (null,'haha',35,'class2305');
Query OK, 1 row affected (0.01 sec)

mysql> select * from test016  ;
+----+------+------+-----------+
| id | name | age  | class     |
+----+------+------+-----------+
|  7 | haha |   35 | class2305 |
+----+------+------+-----------+
1 row in set (0.00 sec)



mysql> alter table test016 drop id;
Query OK, 1 row affected (0.32 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from test016  ;
+------+------+-----------+
| name | age  | class     |
+------+------+-----------+
| haha |   35 | class2305 |
+------+------+-----------+
1 row in set (0.00 sec)

mysql> alter table test016 add
    -> id int primary key  auto_increment first;
Query OK, 0 rows affected (0.40 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> insert into test016  values (null,'haha',35,'class2305');
Query OK, 1 row affected (0.00 sec)

mysql> select * from test016  ;
+----+------+------+-----------+
| id | name | age  | class     |
+----+------+------+-----------+
|  1 | haha |   35 | class2305 |
|  2 | haha |   35 | class2305 |
+----+------+------+-----------+
2 rows in set (0.00 sec)

外键

请添加图片描述

建员工和工资表，指定外键工资列的id与员工表的对应，同步。

mysql> create table test017 (
    -> name char(10),
    -> id int(10) primary key auto_increment)
    -> engine=innodb;
Query OK, 0 rows affected, 1 warning (0.22 sec)

mysql> desc test017;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| name  | char(10) | YES  |     | NULL    |                |
| id    | int(10)  | NO   | PRI | NULL    | auto_increment |
+-------+----------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> insert into  test017(name)  values ('bob'),('tom'),('lisi');
Query OK, 3 rows affected (0.09 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from test017;
+------+----+
| name | id |
+------+----+
| bob  |  1 |
| tom  |  2 |
| lisi |  3 |
+------+----+
3 rows in set (0.00 sec)


1、指定外键：
mysql> alter table test017 rename test017_yg;
Query OK, 0 rows affected (0.05 sec)

mysql> create table test017_gz (
    -> gz_id int,
    -> pay float(8,2),
    -> foreign key(gz_id)  references test017_yg(id) on update cascade  on delete cascade
    -> )engine=innodb;
Query OK, 0 rows affected, 1 warning (0.26 sec)


2、查看结构：
mysql> desc test017_gz;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| gz_id | int(11)    | YES  | MUL | NULL    |       |
| pay   | float(8,2) | YES  |     | NULL    |       |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.01 sec)



mysql> desc test017_yg;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| name  | char(10) | YES  |     | NULL    |                |
| id    | int(10)  | NO   | PRI | NULL    | auto_increment |
+-------+----------+------+-----+---------+----------------+
2 rows in set (0.01 sec)

验证是否有外键：

mysql> show create table test017_gz\G;

*************************** 1. row ***************************
       Table: test017_gz
Create Table: CREATE TABLE `test017_gz` (
  `gz_id` int(11) DEFAULT NULL,
  `pay` float(8,2) DEFAULT NULL,
  KEY `gz_id` (`gz_id`),
  CONSTRAINT `test017_gz_ibfk_1` FOREIGN KEY (`gz_id`) REFERENCES `test017_yg` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.09 sec)

ERROR:
No query specified

添加不在条件范围内的，会报错！！

mysql> select * from test017_yg;
+------+----+
| name | id |
+------+----+
| bob  |  1 |
| tom  |  2 |
| lisi |  3 |
+------+----+
3 rows in set (0.00 sec)

mysql> insert into test017_gz values (4,15566);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`db1`.`test017_gz`, CONSTRAINT `test017_gz_ibfk_1` FOREIGN KEY (`gz_id`) REFERENCES `test017_yg` (`id`) ON DELETE CASCADE ON UPDATE CASCADE)
mysql>

验证，同步删除的效果

mysql> delete from test017_yg where id=2;
Query OK, 1 row affected (0.03 sec)

mysql> select * from test017_yg;
+------+----+
| name | id |
+------+----+
| bob  |  1 |
| lisi |  3 |
+------+----+
2 rows in set (0.00 sec)

mysql> select * from test017_gz;
+-------+----------+
| gz_id | pay      |
+-------+----------+
|  NULL | 11863.55 |
|  NULL | 17580.85 |
|  NULL | 16455.66 |
|     1 | 15600.00 |
|     3 | 15566.00 |
+-------+----------+
5 rows in set (0.00 sec)

修改，同步效果

mysql> update test017_yg set id=7 where name='bob';
Query OK, 1 row affected (0.07 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from test017_yg;
+------+----+
| name | id |
+------+----+
| lisi |  3 |
| bob  |  7 |
+------+----+
2 rows in set (0.00 sec)

mysql> select * from test017_gz;
+-------+----------+
| gz_id | pay      |
+-------+----------+
|  NULL | 11863.55 |
|  NULL | 17580.85 |
|  NULL | 16455.66 |
|     7 | 15600.00 |
|     3 | 15566.00 |
+-------+----------+
5 rows in set (0.00 sec)

删外键：

mysql> alter table test017_gz drop foreign key test017_gz_ibfk_1;
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show create table test017_gz;
+------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table      | Create Table                                                                                                                                                                        |
+------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test017_gz | CREATE TABLE `test017_gz` (
  `gz_id` int(11) NOT NULL,
  `pay` float(8,2) DEFAULT NULL,
  PRIMARY KEY (`gz_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> insert into test017_gz values(9,45222);
Query OK, 1 row affected (0.00 sec)

mysql> select * from test017_gz;
+-------+----------+
| gz_id | pay      |
+-------+----------+
|     3 | 45222.00 |
|     7 | 45222.00 |
|     9 | 45222.00 |
+-------+----------+
3 rows in set (0.00 sec)

mysql> select * from test017_yg;
+------+----+
| name | id |
+------+----+
| lisi |  3 |
| bob  |  7 |
+------+----+
2 rows in set (0.00 sec)

扩展，外键可存空值，这于表的设计思路不符，需要进行控制。把目标外键设置为主键，可避免为空和重复。

mysql> delete from test017_gz;
Query OK, 5 rows affected (0.05 sec)

mysql> desc test017_gz;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| gz_id | int(11)    | YES  | MUL | NULL    |       |
| pay   | float(8,2) | YES  |     | NULL    |       |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> alter table test017_gz add primary key(gz_id);
Query OK, 0 rows affected (0.22 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show create table test017_gz;
+------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table      | Create Table                                                                                                                                                                                                                                                                                                   |
+------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test017_gz | CREATE TABLE `test017_gz` (
  `gz_id` int(11) NOT NULL,
  `pay` float(8,2) DEFAULT NULL,
  PRIMARY KEY (`gz_id`),
  CONSTRAINT `test017_gz_ibfk_1` FOREIGN KEY (`gz_id`) REFERENCES `test017_yg` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

验证：

mysql> insert into test017_gz values(7,45222);
Query OK, 1 row affected (0.08 sec)

mysql> insert into test017_gz values(null,45222);
ERROR 1048 (23000): Column 'gz_id' cannot be null
mysql> insert into test017_gz values(3,45222);
Query OK, 1 row affected (0.01 sec)

mysql> select * from test017_gz;
+-------+----------+
| gz_id | pay      |
+-------+----------+
|     3 | 45222.00 |
|     7 | 45222.00 |
+-------+----------+
2 rows in set (0.00 sec)