一、概览

为了防止不符合规范的数据进入数据库，在用户对数据进行插入、修改、删除等操作时，DBMS自动按照一定的约束条件对数据进行监测，使不符合规范的数据不能进入数据库，以确保数据库中存储的数据正确、有效、相容。

约束条件与数据类型的宽度一样，都是可选参数，主要分为以下几种：

约束条件	解释
NOT NULL	非空约束，指定某列不能为空；
UNIQUE	唯一约束，指定某列或者几列组合不能重复
PRIMARY KEY	主键，指定该列的值可以唯一地标识该列记录
FOREIGN KEY	外键，指定该行记录从属于主表中的一条记录，主要用于参照完整性

二、NOT NULL

是否可空，null表示空，非字符串

not null - 不可空

null - 可空

2.1 not null实例

MysqL> create table t12 (id int not null);
Query OK,0 rows affected (0.02 sec)

MysqL> select * from t12;
Empty set (0.00 sec)

MysqL> desc t12;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
row in set (0.00 sec)

#不能向id列插入空元素。 
MysqL> insert into t12 values (null);
ERROR 1048 (23000): Column 'id' cannot be null

MysqL> insert into t12 values (1);
Query OK,1 row affected (0.01 sec)

@H_403_116@2.2 DEFAULT

我们约束某一列不为空，如果这一列中经常有重复的内容，就需要我们频繁的插入，这样会给我们的操作带来新的负担，于是就出现了默认值的概念。

默认值，创建列时可以指定默认值，当插入数据时如果未主动设置，则自动添加默认值。

2.3 not null + default 示例

MysqL> create table t13 (id1 int not null,id2 int not null default 222);
Query OK,0 rows affected (0.01 sec)

MysqL> desc t13;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id1   | int(11) | NO   |     | NULL    |       |
| id2   | int(11) | NO   |     | 222     |       |
+-------+---------+------+-----+---------+-------+
rows in set (0.01 sec)

# 只向id1字段添加值，会发现id2字段会使用默认值填充
MysqL> insert into t13 (id1) values (111);
Query OK,1 row affected (0.00 sec)

MysqL> select * from t13;
+-----+-----+
| id1 | id2 |
+-----+-----+
| 111 | 222 |
+-----+-----+
row in set (0.00 sec)

# id1字段不能为空，所以不能单独向id2字段填充值；
MysqL> insert into t13 (id2) values (223);
ERROR 1364 (HY000): Field 'id1' doesn't have a default value

# 向id1，id2中分别填充数据，id2的填充数据会覆盖默认值
MysqL> insert into t13 (id1,id2) values (112,223);
Query OK,1 row affected (0.00 sec)

MysqL> select * from t13;
+-----+-----+
| id1 | id2 |
+-----+-----+
| 111 | 222 |
| 112 | 223 |
+-----+-----+
rows in set (0.00 sec)

2.4 not null不生效

设置严格模式：
    不支持对not null字段插入null值
    不支持对自增长字段插入”值
    不支持text字段有默认值

直接在MysqL中生效(重启失效):
MysqL>set sql_mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION";

配置文件添加(永久失效)：
sql-mode="STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION"

三、UNIQUE

唯一约束，指定某列或者几列组合不能重复。

3.1 unique示例

方法一：
create table department1(
id int,name varchar(20) unique,comment varchar(100)
);


方法二：
create table department2(
id int,name varchar(20),comment varchar(100),unique(name)
);


MysqL> insert into department1 values(1,'IT','技术');
Query OK,1 row affected (0.00 sec)
MysqL> insert into department1 values(1,'技术');
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'

3.2 not null 和 unique 的结合

MysqL> create table t1(id int not null unique);
Query OK,0 rows affected (0.02 sec)

MysqL> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
row in set (0.00 sec)

3.3 联合唯一

create table service(
id int primary key auto_increment,host varchar(15) not null,port int not null,unique(host,port) #联合唯一
);

MysqL> insert into service values
    -> (1,'Nginx','192.168.0.10',80),-> (2,'haproxy','192.168.0.20',-> (3,'MysqL','192.168.0.30',3306)
    -> ;
Query OK,3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

MysqL> insert into service(name,host,port) values('Nginx',80);
ERROR 1062 (23000): Duplicate entry '192.168.0.10-80' for key 'host'

四、PRIMARY KEY

主键为了保证表中的每一条数据的该字段都是表格中的唯一值。换言之，它是用来独一无二地确认一个表格中的每一行数据。
主键可以包含一个字段或多个字段。当主键包含多个栏位时，称为组合键 (Composite Key),也可以叫联合主键。
主键可以在建置新表格时设定 (运用 CREATE TABLE 语句)，或是以改变现有的表格架构方式设定 (运用 ALTER TABLE)。
主键必须唯一，主键值非空；可以是单一字段，也可以是多字段组合。

4.1 单字段主键

============单列做主键===============
#方法一：not null+unique
create table department1(
id int not null unique,#主键
name varchar(20) not null unique,comment varchar(100)
);

MysqL> desc department1;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | NO   | UNI | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

#方法二：在某一个字段后用primary key
create table department2(
id int primary key,#主键
name varchar(20),comment varchar(100)
);

MysqL> desc department2;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.00 sec)

#方法三：在所有字段后单独定义primary key
create table department3(
id int,primary key(id); #创建主键并为其命名pk_name

MysqL> desc department3;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

# 方法四：给已经建成的表添加主键约束
MysqL> create table department4(
    -> id int,-> name varchar(20),-> comment varchar(100));
Query OK,0 rows affected (0.01 sec)

MysqL> desc department4;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | YES  |     | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

MysqL> alter table department4 modify id int primary key;
Query OK,0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

MysqL> desc department4;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

4.2 多字段主键

==================多列做主键================
create table service(
ip varchar(15),port char(5),service_name varchar(10) not null,primary key(ip,port)
);


MysqL> desc service;
+--------------+-------------+------+-----+---------+-------+
| Field        | Type        | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| ip           | varchar(15) | NO   | PRI | NULL    |       |
| port         | char(5)     | NO   | PRI | NULL    |       |
| service_name | varchar(10) | NO   |     | NULL    |       |
+--------------+-------------+------+-----+---------+-------+
rows in set (0.00 sec)

MysqL> insert into service values
    -> ('172.16.45.10','3306','MysqLd'),-> ('172.16.45.11','mariadb')
    -> ;
Query OK,2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

MysqL> insert into service values ('172.16.45.10','Nginx');
ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY'

4.3 AUTO_INCREMENT

约束字段为自动增长，被约束的字段必须同时被key约束

4.3.1 设置auto_increment

#不指定id，则自动增长
create table student(
id int primary key auto_increment,sex enum('male','female') default 'male'
);

MysqL> desc student;
+-------+-----------------------+------+-----+---------+----------------+
| Field | Type                  | Null | Key | Default | Extra          |
+-------+-----------------------+------+-----+---------+----------------+
| id    | int(11)               | NO   | PRI | NULL    | auto_increment |
| name  | varchar(20)           | YES  |     | NULL    |                |
| sex   | enum('male','female') | YES  |     | male    |                |
+-------+-----------------------+------+-----+---------+----------------+
MysqL> insert into student(name) values
    -> ('nick'),-> ('tank')
    -> ;

MysqL> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | nick | male |
|  2 | tank | male |
+----+------+------+


#也可以指定id
MysqL> insert into student values(4,'asb','female');
Query OK,1 row affected (0.00 sec)

MysqL> insert into student values(7,'wsb',1 row affected (0.00 sec)

MysqL> select * from student;
+----+------+--------+
| id | name | sex    |
+----+------+--------+
|  1 | nick | male   |
|  2 | tank | male   |
|  4 | asb  | female |
|  7 | wsb  | female |
+----+------+--------+


#对于自增的字段，在用delete删除后，再插入值，该字段仍按照删除前的位置继续增长
MysqL> delete from student;
Query OK,4 rows affected (0.00 sec)

MysqL> select * from student;
Empty set (0.00 sec)

MysqL> insert into student(name) values('ysb');
MysqL> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  8 | ysb  | male |
+----+------+------+

#应该用truncate清空表，比起delete一条一条地删除记录，truncate是直接清空表，在删除大表时用它
MysqL> truncate student;
Query OK,0 rows affected (0.01 sec)

MysqL> insert into student(name) values('nick');
Query OK,1 row affected (0.01 sec)

MysqL> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | nick | male |
+----+------+------+
row in set (0.00 sec)

4.4 offset偏移量（了解）

#在创建完表后，修改自增字段的起始值
MysqL> create table student(
    -> id int primary key auto_increment,-> sex enum('male','female') default 'male'
    -> );

MysqL> alter table student auto_increment=3;

MysqL> show create table student;
.......
ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8

MysqL> insert into student(name) values('nick');
Query OK,1 row affected (0.01 sec)

MysqL> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  3 | nick | male |
+----+------+------+
row in set (0.00 sec)

MysqL> show create table student;
.......
ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8


#也可以创建表时指定auto_increment的初始值，注意初始值的设置为表选项，应该放到括号外
create table student(
id int primary key auto_increment,'female') default 'male'
)auto_increment=3;




#设置步长
sqlserver：自增步长
    基于表级别
    create table t1（
        id int。。。
    ）engine=innodb,auto_increment=2 步长=2 default charset=utf8

MysqL自增的步长：
    show session variables like 'auto_inc%';
    
    #基于会话级别
    set session auth_increment_increment=2 #修改会话级别的步长

    #基于全局级别的
    set global auth_increment_increment=2 #修改全局级别的步长（所有会话都生效）


#！！！注意了注意了注意了！！！
If the value of auto_increment_offset is greater than that of auto_increment_increment,the value of auto_increment_offset is ignored. 
翻译：如果auto_increment_offset的值大于auto_increment_increment的值，则auto_increment_offset的值会被忽略 ，这相当于第一步步子就迈大了，扯着了蛋
比如：设置auto_increment_offset=3，auto_increment_increment=2




MysqL> set global auto_increment_increment=5;
Query OK,0 rows affected (0.00 sec)

MysqL> set global auto_increment_offset=3;
Query OK,0 rows affected (0.00 sec)

MysqL> show variables like 'auto_incre%'; #需要退出重新登录
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 1     |
| auto_increment_offset    | 1     |
+--------------------------+-------+



create table student(
id int primary key auto_increment,'female') default 'male'
);

MysqL> insert into student(name) values('nick1'),('nick2'),('nick3');
MysqL> select * from student;
+----+-------+------+
| id | name  | sex  |
+----+-------+------+
|  3 | nick1 | male |
|  8 | nick2 | male |
| 13 | nick3 | male |
+----+-------+------+

步长:auto_increment_increment,起始偏移量:auto_increment_offset

五、FOREIGN KEY

多表：

假设我们要描述所有公司的员工，需要描述的属性有这些：工号姓名部门

公司有3个部门，但是有1个亿的员工，那意味着部门这个字段需要重复存储，部门名字越长，越浪费

解决方法：我们完全可以定义一个部门表然后让员工信息表关联该表，如何关联，即foreign key

5.1 创造外键的条件

MysqL> create table departments (dep_id int(4),dep_name varchar(11));
Query OK,0 rows affected (0.02 sec)

MysqL> desc departments;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| dep_id   | int(4)      | YES  |     | NULL    |       |
| dep_name | varchar(11) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
rows in set (0.00 sec)

# 创建外键不成功
MysqL> create table staff_info (s_id int,dep_id int,foreign key(dep_id) references departments(dep_id));
ERROR 1215 (HY000): Cannot add foreign key 

# 设置dep_id非空，仍然不能成功创建外键
MysqL> alter table departments modify dep_id int(4) not null;
Query OK,0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

MysqL> desc departments;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| dep_id   | int(4)      | NO   |     | NULL    |       |
| dep_name | varchar(11) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
rows in set (0.00 sec)

MysqL> create table staff_info (s_id int,foreign key(dep_id) references departments(dep_id));
ERROR 1215 (HY000): Cannot add foreign key constraint

# 当设置字段为unique唯一字段时，设置该字段为外键成功
MysqL> alter table departments modify dep_id int(4) unique;
Query OK,0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

MysqL> desc departments;                                                                                                       +----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| dep_id   | int(4)      | YES  | UNI | NULL    |       |
| dep_name | varchar(11) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
rows in set (0.01 sec)

MysqL> create table staff_info (s_id int,foreign key(dep_id) references departments(dep_id));
Query OK,0 rows affected (0.02 sec)

5.2 外键操作示例

#表类型必须是innodb存储引擎，且被关联的字段，即references指定的另外一个表的字段，必须保证唯一
create table department(
id int primary key,name varchar(20) not null
)engine=innodb;

#dpt_id外键，关联父表（department主键id），同步更新，同步删除
create table employee(
id int primary key,name varchar(20) not null,dpt_id int,foreign key(dpt_id)
references department(id)
on delete cascade  # 级连删除
on update cascade # 级连更新
)engine=innodb;


#先往父表department中插入记录
insert into department values
(1,'教质部'),(2,'技术部'),(3,'人力资源部');


#再往子表employee中插入记录
insert into employee values
(1,'jason',1),'oscar',2),'nick',(4,'tank',(5,'mac',3),(6,'李沁洋',(7,'皮卡丘',(8,'程咬金',(9,'程咬银',3)
;


#删父表department，子表employee中对应的记录跟着删
MysqL> delete from department where id=2;
Query OK,1 row affected (0.00 sec)

MysqL> select * from employee;
+----+-----------+--------+
| id | name      | dpt_id |
+----+-----------+--------+
|  1 | jason      |      1 |
|  5 | mac     |      3 |
|  6 | 李沁洋    |      3 |
|  7 | 皮卡丘    |      3 |
|  8 | 程咬金    |      3 |
|  9 | 程咬银    |      3 |
+----+-----------+--------+
rows in set (0.00 sec)


#更新父表department，子表employee中对应的记录跟着改
MysqL> update department set id=2 where id=3;
Query OK,1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

MysqL> select * from employee;
+----+-----------+--------+
| id | name      | dpt_id |
+----+-----------+--------+
|  1 | jason      |      1 |
|  5 | mac     |      2 |
|  6 | 李沁洋    |      2 |
|  7 | 皮卡丘    |      2 |
|  8 | 程咬金    |      2 |
|  9 | 程咬银    |      2 |
+----+-----------+--------+
rows in set (0.00 sec)

5.3 on delete(了解)

. cascade方式
在父表上update/delete记录时，同步update/delete掉子表的匹配记录 

. set null方式
在父表上update/delete记录时，将子表上匹配记录的列设为null
要注意子表的外键列不能为not null  

. No action方式
如果子表中有匹配的记录,则不允许对父表对应候选键进行update/delete操作  

. Restrict方式
同no action,都是立即检查外键约束

. Set default方式
父表有变更时,子表将外键列设置成一个默认的值 但Innodb不能识别

MySQL表的完整性约束