数据库操作之——约束

一 约束的种类

　　primary key

　　unique key

　　not null

　　zerofill

　　default

　　auto_increment

　　foreign key

　　unsigned　　

　　

　　1 default。 默认

　　示例一

mysql> create table t7(id int ,sex char(6) default 'male');
Query OK, 0 rows affected (0.28 sec)

mysql> desc t7;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES  |     | NULL    |       |
| sex   | char(6) | YES  |     | male    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.01 sec)

 　　示例二

mysql> create table t1(id int primary key auto_increment,name char(6),sex char(5) default 'male');
Query OK, 0 rows affected (0.48 sec)

mysql> insert t1(name) values
    -> ('alex');
Query OK, 1 row affected (0.30 sec)

mysql> select * from t1;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | alex | male |
+----+------+------+
1 row in set (0.00 sec)

　　2 auto_increment   自增

　　针对所有表的设置：

　　设置自增的步长：auto_increment_increment

　　设置自增的初始值：auto_increment_offset

　　

　　针对单个表的设置：

　　auto_increment

　　

　　查看回话级别变量

　　show session variables like 'auto_inc%';

　　查看全局级别变量

　　show global variables like 'auto_inc%';

　　设置基于回话级别：

　　set session auto_increment_increment=2

　　设置基于全局级别

　　set global auto_increment_increment=2

　　PS：

　　　　如果自增的步长小于自增的初始值，则自增的初始值会被忽略！即自增的初始值一定要小于等于自增的步长！

　　　　所以你会发现这个并没有多大卵用。

　　设置完以后，不能生效。退出，重新登录，才能生效。

　　示例 1：

　　针对所有表的设置

mysql> set global auto_increment_increment=2;
Query OK, 0 rows affected (0.00 sec)

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

mysql> q
Bye

C:Userszuo>mysql
Welcome to the MySQL monitor.  Commands end with ; or g.
Your MySQL connection id is 19

mysql> create table t2(id int primary key auto_increment,name char(6));
Query OK, 0 rows affected (0.28 sec)

mysql> insert t2(name) values
    -> ('alex'),
    -> ('egon'),
    -> ('wupeiqi');
Query OK, 3 rows affected, 1 warning (0.06 sec)
Records: 3  Duplicates: 0  Warnings: 1

mysql> select * from t2;
+----+--------+
| id | name   |
+----+--------+
|  1 | alex   |
|  3 | egon   |
|  5 | wupeiq |
+----+--------+
3 rows in set (0.00 sec)

mysql> show global variables like 'auto_inc%';
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 2     |
| auto_increment_offset    | 3     |
+--------------------------+-------+
2 rows in set (0.00 sec)

　　因为自增的步长小于自增的初始值，所以自增的初始值被忽略！

　　

　　示例 2：

　　针对单个表的设置：

mysql> create table t3(id int primary key auto_increment,name char(4))auto_increment=10;
Query OK, 0 rows affected (0.26 sec)

mysql> insert t3(name) values
    -> ('egon'),
    -> ('alex');
Query OK, 2 rows affected (0.06 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from t3;
+----+------+
| id | name |
+----+------+
| 10 | egon |
| 11 | alex |
+----+------+
2 rows in set (0.00 sec)

　　3 foreign key 外键

　　外键具有保持数据完整性和一致性的机制，目前MySQL只在InnoDB引擎下支持。

　　B 存在外键 b_f_k，以 A 表的 a_k 作为参照列，则 A 为主表，B 为从表，A 中某记录更新或删除时将会联动 B 中外键与其关联对应的记录做更新或删除操作。

　　举个例子：员工表、部门表。关系是：一个员工属于一个部门，一个部门有多个员工在这里，员工表中的部门编号就是外键。部门表是主键表（主表）员工表是外键表（从表）。

　　注意：

　　先创建主表，然后创建从表。

　　主表中的操作（删除，修改）会影响到从表。

　　示例：

　　多对一的情形：

 create table dep(id int primary key auto_increment,name char(7));
Query OK, 0 rows affected (0.26 sec)

mysql> insert dep(name) values
    -> ('IT'),
    -> ('xx'),
    -> ('yy');
Query OK, 3 rows affected (0.06 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> create table emp(id int primary key auto_increment,name char(6),dep_id int, foreign key(dep_id) references dep(id) on delete cascade on update cascade);
Query OK, 0 rows affected (0.26 sec)

mysql> insert emp(name,dep_id) values
    -> ('egon',1),
    -> ('alex',1),
    -> ('wupeiqi',1),
    -> ('sb',2);
Query OK, 4 rows affected, 1 warning (0.05 sec)
Records: 4  Duplicates: 0  Warnings: 1

mysql> select * from emp;
+----+--------+--------+
| id | name   | dep_id |
+----+--------+--------+
|  1 | egon   |      1 |
|  2 | alex   |      1 |
|  3 | wupeiq |      1 |
|  4 | sb     |      2 |
+----+--------+--------+
4 rows in set (0.00 sec)

mysql> show create table emp;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                                     |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| emp   | CREATE TABLE `emp` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` char(6) DEFAULT NULL,
  `dep_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `dep_id` (`dep_id`),
  CONSTRAINT `emp_ibfk_1` FOREIGN KEY (`dep_id`) REFERENCES `dep` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> delete from dep where id =1;
Query OK, 1 row affected (0.06 sec)

mysql> select * from emp;
+----+------+--------+
| id | name | dep_id |
+----+------+--------+
|  4 | sb   |      2 |
+----+------+--------+
1 row in set (0.00 sec)

　　多对多的情形：

　　就没有主表，从表之分了。怎么办呢，需要通过一个第三方的中介，额外创建一个的table。

　　

mysql> create table class(id int primary key auto_increment,name char(6),stu_id int);
Query OK, 0 rows affected (0.25 sec)

mysql> create table student(id int primary key auto_increment,name char(6),cla_id int );
Query OK, 0 rows affected (0.24 sec)

mysql> create table stu2cla(id int primary key auto_increment,stu_id int,cla_id int,foreign key(stu_id) references student(id) on delete cascade on update cascade,foreign key(cla_id) references class(id) on delete cascade on update cascade);
Query OK, 0 rows affected (0.30 sec)

mysql> insert class(name,stu_id) values
    -> ('python',1),
    -> ('python',2),
    -> ('python',3),
    -> ('python',4),
    -> ('linux',5),
    -> ('linux',6);
Query OK, 6 rows affected (0.05 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> insert student(name,cla_id) values
    -> ('egon',1),
    -> ('alex',2),
    -> ('wupeiqi',3),
    -> ('yuanhao',4),
    -> ('egon',5),
    -> ('egon',6);
Query OK, 6 rows affected, 2 warnings (0.06 sec)
Records: 6  Duplicates: 0  Warnings: 2

mysql> insert stu2cla(stu_id,cla_id) values
    -> (1,2),
    -> (2,2),
    -> (2,1);
Query OK, 3 rows affected (0.30 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> show create table stu2cla;
+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| stu2cla | CREATE TABLE `stu2cla` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `stu_id` int(11) DEFAULT NULL,
  `cla_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `stu_id` (`stu_id`),
  KEY `cla_id` (`cla_id`),
  CONSTRAINT `stu2cla_ibfk_1` FOREIGN KEY (`stu_id`) REFERENCES `student` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `stu2cla_ibfk_2` FOREIGN KEY (`cla_id`) REFERENCES `class` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 |
+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)