MySQL的约束

MySQL中约束保存在information_schema数据库的table_constraints中，可以通过该表查询约束信息；

约束主要完成对数据的检验，保证数据库数据的完整性，如果有相互依赖数据，保证该数据不被删除，因此约束也叫完整性约束。

什么是完整性？完整性是指数据库中存放的数据是有意义的、正确的。

什么是约束？为了保证数据的正确性和相容性，对关系模型提出来的某些约束条件或者规则。

1.默认值（default）：当用户向表中插入数据时，指定了该字段的那么就插入该值，否则就插入默认值。

mysql> create table test(id int(6) primary key auto_increment,name varchar(10) not null default '未知',addr varchar(24) not null default '未知');
Query OK, 0 rows affected (0.07 sec)

mysql> desc test;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(6)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(10) | NO   |     | 未知    |                |
| addr  | varchar(24) | NO   |     | 未知    |                |
+-------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> insert into test values(1,'科比','湖北罗田');
Query OK, 1 row affected (0.01 sec)

mysql> insert into test(name) values('毛线');
Query OK, 1 row affected (0.00 sec)

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

mysql> select * from test;
+----+--------+--------------+
| id | name   | addr         |
+----+--------+--------------+
|  1 | 科比   | 湖北罗田     |
|  2 | 毛线   | 未知         |
|  3 | 未知   | 未知         |
+----+--------+--------------+
3 rows in set (0.00 sec)

2.非空（not null）：所谓的非空就是字段的名不能输入为null（大小写无所谓），非空不是说不能没有值，也不是说不能为空格。

mysql> create table weiyi(id int(4) primary key,name char(10) not null);
Query OK, 0 rows affected (0.01 sec)

mysql> desc weiyi;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(4)   | NO   | PRI | NULL    |       |
| name  | char(10) | NO   |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into weiyi(id) values(1);  #指定非空可以不输
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> insert into weiyi(id,name) values(2,' ');  #可以为空格
Query OK, 1 row affected (0.01 sec)

mysql> insert into weiyi(id,name) values(3,Null);
ERROR 1048 (23000): Column 'name' cannot be null  #不能为空

mysql> create table weiyi2(id int(4),name char(10));
Query OK, 0 rows affected (0.03 sec)

mysql> insert into weiyi2(id,name) values(1,NULL);
Query OK, 1 row affected (0.05 sec)

3.唯一unique：在这个表格中，设置了unique的这个字段，所有的值只能出现一次。

mysql> create table test_1(id int(2),name char(10) unique);
Query OK, 0 rows affected (0.02 sec)

mysql> insert into test_1 values(1,'kebi');
Query OK, 1 row affected (0.01 sec)

mysql> insert into test_1 values(2,'kebi');
ERROR 1062 (23000): Duplicate entry 'kebi' for key 'name'

4.自增（auto_increment）

建立自增属性有两点要求：

1）字段上要有唯一性索引或者主键

mysql> create table zizeng1(id int(4) auto_increment,name char(10));
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key

2）该字段必须是数值型

mysql> create table zizeng1(id char(4) primary key auto_increment,name char(10));
ERROR 1063 (42000): Incorrect column specifier for column 'id'

 自增的表现形式：

在不指定具体的值的时候，设置了自增的字段添加的值会在上一条记录上加一。

mysql> select * from auto;
+----+------+
| id | name |
+----+------+
|  3 | kebi |
+----+------+
1 row in set (0.00 sec)

mysql> insert into auto(name) values('maoxian');
Query OK, 1 row affected (0.01 sec)

mysql> select * from auto;
+----+---------+
| id | name    |
+----+---------+
|  3 | kebi    |
|  4 | maoxian |
+----+---------+
2 rows in set (0.00 sec)

如果添加的值比之前的字段值小，会自动往前面添加。

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

mysql> select * from auto;
+----+----------+
| id | name     |
+----+----------+
|  1 | xiaoniao |
|  3 | kebi     |
|  4 | maoxian  |
+----+----------+
3 rows in set (0.00 sec)

delete删除值，之后自增的值还是会在之前最大的值的基础上增加。

mysql> insert into auto values(1000,'huangyu');
Query OK, 1 row affected (0.00 sec)

mysql> select * from auto;
+------+----------+
| id   | name     |
+------+----------+
|    1 | xiaoniao |
|    3 | kebi     |
|    4 | maoxian  |
| 1000 | huangyu  |
+------+----------+
4 rows in set (0.00 sec)

mysql> delete from auto;
Query OK, 4 rows affected (0.00 sec)

mysql> select * from auto;
Empty set (0.00 sec)    #空的

mysql> insert into auto(name) values('maoxian');
Query OK, 1 row affected (0.01 sec)

mysql> select * from auto;
+------+---------+
| id   | name    |
+------+---------+
| 1001 | maoxian |    #从1001开始
+------+---------+
1 row in set (0.00 sec)

trancate会清空表，而且也会清空自增的效果。

mysql> select * from auto;
+------+---------+
| id   | name    |
+------+---------+
| 1001 | maoxian |
| 1002 | zidong  |
| 1003 | zihao   |
+------+---------+
3 rows in set (0.00 sec)

mysql> truncate table auto;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from auto;
Empty set (0.00 sec)

mysql> insert into auto(name) values('zihao');
Query OK, 1 row affected (0.01 sec)

mysql> select * from auto;
+----+-------+
| id | name  |
+----+-------+
|  1 | zihao |
+----+-------+
1 row in set (0.00 sec)

5.主键（primary key）

主键是表中的特殊字段，这个字段能够唯一标识表中的每一条记录。

一张表最多只能有一个主键，主键需要满足的条件：非空且唯一。

主键常常与外键构成参照完整性的约束，防止出现数据不一致。

主键可以保证记录的唯一和主键域非空，数据库管理系统对于主键自动生成唯一索引，所以主键也是一个特殊的索引。

mysql> create table auto(id int(4) primary key auto_increment,name varchar(10));

还有一点需要提出来的是“联合主键”，多个字段的值只要不都不变，就不会报错。

mysql> create table test(id int(4),name char(10),primary key(id,name));
Query OK, 0 rows affected (0.01 sec)
mysql> select * from test;
+----+-------+
| id | name  |
+----+-------+
|  0 |       |
|  0 | kebi  |
|  0 | zihao |
+----+-------+

mysql> create table test2(id int(4) primary key,name char(10));
Query OK, 0 rows affected (0.03 sec)
mysql> insert into test2(name) values('maoxian');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> insert into test2(name) values('huangyu');
ERROR 1062 (23000): Duplicate entry '0' for key 'PRIMARY'  #不能满足唯一性

6.外键 （foreign key）

外键是用于建立和加强两个表数据之间的链接的一列或多列。外键约束主要用来维护两个表之间数据的一致性。简而言之，表的外键就是另一张表的主键，外键将两表联系起来。

创建外键的条件：

（1）存储引擎是innodb，存储引擎不是innodb创建是不会报错，但是数据之间失去了关联。

mysql> create table mem(id int,name varchar(20),foreign key(id) references class(id)) engine=myisam;
Query OK, 0 rows affected (0.09 sec)
mysql> select * from class;
+----+-------+
| id | name  |
+----+-------+
|  2 | zihao |
+----+-------+
1 row in set (0.00 sec)
mysql> insert into mem values(3,'kebi');
Query OK, 1 row affected (0.00 sec)

（2）相关联字段数据类型要一致；

mysql> create table mem(id int(20),name varchar(20),foreign key(id) references class(id));
Query OK, 0 rows affected (0.10 sec)

mysql> select * from class;
+----+-------+
| id | name  |
+----+-------+
|  2 | zihao |
+----+-------+
1 row in set (0.00 sec)

mysql> insert into class values(3,'zihao');
Query OK, 1 row affected (0.00 sec)

（3）最好在外键列上建索引（目的就是为了减少扫面范围，不创建也可以，只是影响性能）。

外键的表现形式：

（1）外键列的值依赖主键列的值，如果主键列中没有，那么外键也不能添加

（2）不能删除主键列中被外键列依赖的值。

mysql> create table class(id int primary key,name varchar(20));
Query OK, 0 rows affected (0.08 sec)

mysql> create table mem(id int,name varchar(20),foreign key(id) references class(id));
Query OK, 0 rows affected (0.01 sec)

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

mysql> insert into class values(1,'kebi'),(2,'zihao');
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0

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

mysql> insert into mem values(5,'daipeng');
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`yueshu`.`mem`, CONSTRAINT `mem_ibfk_1` FOREIGN KEY (`id`) REFERENCES `class` (`id`))

mysql> select * from class;
+----+-------+
| id | name  |
+----+-------+
|  1 | kebi  |
|  2 | zihao |
+----+-------+
2 rows in set (0.00 sec)

mysql> select * from mem;
+------+---------+
| id   | name    |
+------+---------+
|    1 | huangyu |
|    2 | daipeng |
+------+---------+
2 rows in set (0.00 sec)

mysql> delete from class where id = 1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`yueshu`.`mem`, CONSTRAINT `mem_ibfk_1` FOREIGN KEY (`id`) REFERENCES `class` (`id`))
mysql> delete from mem where id = 1;
Query OK, 1 row affected (0.00 sec)

mysql> delete from class where id = 1;
Query OK, 1 row affected (0.00 sec)

到这里总是觉得外键没有啥用，反而碍手碍脚。怎样让父表不受子表的约束了？

有两种方法：

（1）删除外键约束

（2）指定级联操作的选项。

on delete cascade:级联删除，父表删除记录，子表随之删除相应的记录。

on update cascade级联更新，父表更改数据，子表也会随之更新，但是父表添加，主表没有反应。

#创建外连接，同时指定级联删除和级联更新
mysql> create table mem(id int,name char(10),index(id),foreign key(id) references class(id) on delete cascade on update cascade);
Query OK, 0 rows affected (0.01 sec)

mysql> select * from class;
+----+----------+
| id | name     |
+----+----------+
|  2 | zihao    |
|  3 | xiaoniao |
+----+----------+
2 rows in set (0.00 sec)
#级联更新
mysql> update class set id = 30 where name = "xiaoniao";
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from mem;
+------+----------+
| id   | name     |
+------+----------+
|    2 | xiaocui  |
|   30 | meiliang |
+------+----------+
2 rows in set (0.00 sec)
#级联删除
mysql> delete from class where id = 2;
Query OK, 1 row affected (0.01 sec)

mysql> select * from mem;
+------+----------+
| id   | name     |
+------+----------+
|   30 | meiliang |
+------+----------+
1 row in set (0.00 sec)

#然而并没有级联更新
mysql> insert into class values(1,'doubi');
Query OK, 1 row affected (0.08 sec)

mysql> select * from mem;
+------+----------+
| id   | name     |
+------+----------+
|   30 | meiliang |
+------+----------+
1 row in set (0.00 sec)

mysql> show create table mem;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                         |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| mem   | CREATE TABLE `mem` (
  `id` int(11) DEFAULT NULL,
  `name` char(10) DEFAULT NULL,
  KEY `id` (`id`),
  CONSTRAINT `mem_ibfk_1` FOREIGN KEY (`id`) REFERENCES `class` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> alter table mem drop foreign key mem_ibfk_1;外键名，在表的属性中查找
Query OK, 1 row affected (0.04 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> show create table mem;
+-------+------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                             |
+-------+------------------------------------------------------------------------------------------------------------------------------------------+
| mem   | CREATE TABLE `mem` (
  `id` int(11) DEFAULT NULL,
  `name` char(10) DEFAULT NULL,
  KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)