mysql表的完整性约束

约束条件与数据类型的宽度一样，都是可选参数

作用：用于保证数据的完整性和一致性
主要分为：

PRIMARY KEY (PK)    标识该字段为该表的主键，可以唯一的标识记录
FOREIGN KEY (FK)    标识该字段为该表的外键
NOT NULL    标识该字段不能为空
UNIQUE KEY (UK)    标识该字段的值是唯一的
AUTO_INCREMENT    标识该字段的值自动增长（整数类型，而且为主键）
DEFAULT    为该字段设置默认值
 
UNSIGNED 无符号
ZEROFILL 使用0填充

　　

1. 是否允许为空，默认NULL，可设置NOT NULL，字段不允许为空，必须赋值
2. 字段是否有默认值，缺省的默认值是NULL，如果插入记录时不给字段赋值，此字段使用默认值
sex enum('male','female') not null default 'male'
age int unsigned NOT NULL default 20 必须为正值（无符号） 不允许为空 默认是20
3. 是否是key
主键 primary key
外键 foreign key
索引 (index,unique...)

　　

NOT NULL

是否可空，null表示空，非字符串
not null - 不可空
null - 可空 

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)

not null示例

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

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)

not null + default 示例

UNIQUE

唯一约束，指定某列或者几列组合不能重复

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,'IT','技术');
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'

unique示例

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)

not null 和unique的结合

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。

============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,
name varchar(20),
comment varchar(100),
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)

单字段主键

==================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','3306','mariadb')
    -> ;
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

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

多字段主键

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

#不指定id，则自动增长
create table student(
id int primary key auto_increment,
name varchar(20),
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
    -> ('egon'),
    -> ('alex')
    -> ;

mysql> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | egon | male |
|  2 | alex | 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','female');
Query OK, 1 row affected (0.00 sec)

mysql> select * from student;
+----+------+--------+
| id | name | sex    |
+----+------+--------+
|  1 | egon | male   |
|  2 | alex | 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('egon');
Query OK, 1 row affected (0.01 sec)

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

设置auto_increment

FOREIKEY

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

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,name varchar(20),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,name varchar(20),dep_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,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id));
Query OK, 0 rows affected (0.02 sec)

创造外键的条件

#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  # 连级删除
on update cascade # 连级更新
)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,'yuan',1),
(2,'nezha',2),
(3,'egon',2),
(4,'alex',2),
(5,'wusir',3),
(6,'李沁洋',3),
(7,'皮卡丘',3),
(8,'程咬金',3),
(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 | yuan      |      1 |
|  5 | wusir     |      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 | yuan      |      1 |
|  5 | wusir     |      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)

外键操作示例