真正约束字段的是数据类型,但是数据类型约束很单一,需要有一些额外的约束,更好的保证数据的合法性,从业务逻辑角度保证数据的正确性。比如说我们的居民身份证,电话号码,都被要求是唯一的,这就需要约束。
空属性约束字段:null(默认)和 not null(不为空)
数据库默认字段基本都是字段为空,但是实际开发时,尽可能保证字段不为空,因为数据为空没办法参与运算。
mysql> select NULL;
+------+
| NULL |
+------+
| NULL |
+------+
1 row in set (0.00 sec)
mysql> select NULL+1;
+--------+
| NULL+1 |
+--------+
| NULL |
+--------+
1 row in set (0.01 sec)
MySQL中的NULL不同于C语言的NULL,C语言中的NULL表示ASCII表中的0,而MySQL中的NULL表示空,不参与任何运算。
案例:
创建一个班级表,包含班级名和班级所在的教室。
站在正常的业务逻辑中:
所以我们在设计数据库表的时候,一定要在表中进行限制,满足上面条件的数据就不能插入到表中。这就是“约束”。
mysql> create table class (
-> class_name varchar(20) not null,
-> class_room varchar(20) not null
-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.03 sec)
mysql> desc class;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| class_name | varchar(20) | NO | | NULL | |
| class_room | varchar(20) | NO | | NULL | |
+------------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
插入数据时,如果没有给班级名或教室名,MySQL就会报错:
mysql> insert into class (class_name) values ('20计算机');
ERROR 1364 (HY000): Field 'class_room' doesn't have a default value
默认值:某一种数据会经常性的出现某个具体的值,可以在一开始就指定好,在需要真实数据的时候,用户可以选择性的使用默认值。
案例:
mysql> create table persons (
-> name varchar(20) not null,
-> age tinyint default 18,
-> sex char(2) default '男'
-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)
mysql> desc persons;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| name | varchar(20) | NO | | NULL | |
| age | tinyint(4) | YES | | 18 | |
| sex | char(2) | YES | | 男 | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql> insert into persons (name) values ('Curry');
Query OK, 1 row affected (0.01 sec)
mysql> select * from persons;
+-------+------+------+
| name | age | sex |
+-------+------+------+
| Curry | 18 | 男 |
+-------+------+------+
1 row in set (0.00 sec)
只有在该字段定义了默认值后,在插入的时候,才可以进行列省略。
列描述:comment,没有实际含义,专门用来描述字段,会根据表创建语句保存。
mysql> create table people (
-> name varchar(20) not null comment '姓名',
-> age tinyint default 18 comment '年龄',
-> sex char(2) default '男' comment '性别'
-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.01 sec)
通过desc people查看不到注释:
mysql> desc people;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| name | varchar(20) | NO | | NULL | |
| age | tinyint(4) | YES | | 18 | |
| sex | char(2) | YES | | 男 | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
通过show create table people可以查看:
mysql> show create table people \G
*************************** 1. row ***************************
Table: people
Create Table: CREATE TABLE `people` (
`name` varchar(20) NOT NULL COMMENT '姓名',
`age` tinyint(4) DEFAULT '18' COMMENT '年龄',
`sex` char(2) DEFAULT '男' COMMENT '性别'
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
数据类型中,整型数据后的长度代表的意义与zerofill紧密相关
mysql> show create table people \G
*************************** 1. row ***************************
Table: people
Create Table: CREATE TABLE `people` (
`name` varchar(20) NOT NULL COMMENT '姓名',
`age` int(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
案例:
mysql> create table table1 ( a int(10) zerofill, b int(10) )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)
mysql> desc table1;
+-------+---------------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------------+------+-----+---------+-------+
| a | int(10) unsigned zerofill | YES | | NULL | |
| b | int(10) | YES | | NULL | |
+-------+---------------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
mysql> insert into table1 (a,b) values (10, 10);
Query OK, 1 row affected (0.01 sec)
mysql> select * from table1;
+------------+------+
| a | b |
+------------+------+
| 0000000010 | 10 |
+------------+------+
1 row in set (0.00 sec)
zerofill属性的作用是如果宽度小于设定的宽度,自动填充0。要注意的是,这只是最后显示的结果,在MySQL中实际存储的还是1。
主键:primary key用来唯一的约束该字段里面的数据,不能重复,不能为空,一张表中最多只能有一个主键;主键所在的列通常是整数类型。
案例:
mysql> create table students (
-> id int unsigned primary key,
-> name varchar(10) not null
-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.01 sec)
mysql> desc students;
+-------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| id | int(10) unsigned | NO | PRI | NULL | |
| name | varchar(10) | NO | | NULL | |
+-------+------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
主键约束:主键对应的字段中不能重复,一旦重复,操作失败。
mysql> insert into students (id,name) values (1, 'Durant');
Query OK, 1 row affected (0.00 sec)
mysql> insert into students (id,name) values (1, 'Curry');
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
删除主键:
alter table 表名 drop primary key
示例:
mysql> alter table students drop primary key;
Query OK, 1 row affected (0.04 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> desc students;
+-------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| id | int(10) unsigned | NO | | NULL | |
| name | varchar(10) | NO | | NULL | |
+-------+------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
当表创建好以后但是没有主键的时候,可以再次追加主键:
alter table 表名 add primary key (字段列表)
示例:
mysql> alter table students add primary key (id);
Query OK, 0 rows affected (0.07 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc students;
+-------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| id | int(10) unsigned | NO | PRI | NULL | |
| name | varchar(10) | NO | | NULL | |
+-------+------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
复合主键:在创建表的时候,在所有字段之后,使用primary key(主键字段列表)来创建主键,如果有多个字段作为主键,可以使用复合主键。
案例:
mysql> create table students (
-> id int unsigned,
-> course char(10) comment '课程代码',
-> score tinyint unsigned comment '分数',
-> primary key(id, course)
-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)
mysql> desc students;
+--------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+-------+
| id | int(10) unsigned | NO | PRI | NULL | |
| course | char(10) | NO | PRI | NULL | |
| score | tinyint(3) unsigned | YES | | NULL | |
+--------+---------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql> insert into students (id,course) values (1, '123');
Query OK, 1 row affected (0.00 sec)
mysql> insert into students (id,course) values (1, '124');
Query OK, 1 row affected (0.00 sec)
mysql> insert into students (id,course) values (1, '123');
ERROR 1062 (23000): Duplicate entry '1-123' for key 'PRIMARY'
auto_increment:当对应的字段,不给值,会自动的被系统触发,系统会从当前字段中已经有的最大值+1操作,得到一个新的不同的值。通常和主键搭配使用,作为逻辑主键。
自增长的特点:
mysql> create table persons (
-> id int unsigned primary key auto_increment,
-> name varchar(10) not null
-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)
mysql> desc persons;
+-------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(10) | NO | | NULL | |
+-------+------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql> insert into persons (name) values ('a');
Query OK, 1 row affected (0.00 sec)
mysql> insert into persons (name) values ('b');
Query OK, 1 row affected (0.00 sec)
mysql> select * from persons;
+----+------+
| id | name |
+----+------+
| 1 | a |
| 2 | b |
+----+------+
2 rows in set (0.00 sec)
一张表中有往往有很多字段需要唯一性,数据不能重复,但是一张表中只能有一个主键:唯一键就可以解决表中有多个字段需要唯一性约束的问题。
唯一键的本质和主键差不多,唯一键允许为空,而且可以多个为空,空字段不做唯一性比较。
关于唯一键和主键的区别:
我们可以简单理解成,主键更多的是标识唯一性的。而唯一键更多的是保证在业务上,不要和别的信息出现重复。
比如说,有一张存放公民信息的表,里面字段有身份证号,姓名,性别,手机号和家庭住址,我们可以使用身份证号作为主键,因为身份证号码具有唯一性,但是由于数据库是具有很强的数据明确性的,不允许有本应该唯一的字段被误操作添加重复的值,所以数据库就需要一种约束,约束公民的其他唯一存在的字段比如手机号不能重复,这个约束就叫做唯一键。
案例:
mysql> create table person (
-> id int unsigned primary key,
-> telephone int unsigned unique not null,
-> sex char(2) default '男',
-> address varchar(20) not null
-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.03 sec)
mysql> desc person;
+-----------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| id | int(10) unsigned | NO | PRI | NULL | |
| telephone | int(10) unsigned | NO | UNI | NULL | |
| sex | char(2) | YES | | 男 | |
| address | varchar(20) | NO | | NULL | |
+-----------+------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
mysql> insert into person (id, telephone, sex, address) values(3306, 8520170, '男', '福建漳州');
Query OK, 1 row affected (0.00 sec)
mysql> insert into person (id, telephone, sex, address) values(8080, 8520170, '男', '福建漳州');
ERROR 1062 (23000): Duplicate entry '8520170' for key 'telephone'
外键用于定义主表和从表之间的关系:外键约束主要定义在从表上,主表则必须是有主键约束或unique约束。当定义外键后,要求外键列数据必须在主表的主键列存在或为null。
对比外键和外键约束:
案例:
创建主表:
mysql> create table Class (
-> id int primary key,
-> class_name varchar(20) not null
-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)
mysql> desc Class;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| class_name | varchar(20) | NO | | NULL | |
+------------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
创建从表:
mysql> create table Stu (
-> id int primary key,
-> name varchar(20) not null,
-> class_id int,
-> foreign key (class_id) references Class(id)
-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.03 sec)
mysql> desc Stu;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | NO | | NULL | |
| class_id | int(11) | YES | MUL | NULL | |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
插入数据:如果插入不存在的班级,就会报错
mysql> insert into Stu (id, name, class_id) values (30, 'Curry', 10);
Query OK, 1 row affected (0.00 sec)
mysql> insert into Stu (id, name, class_id) values (35, 'Durant', 20);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`table_restraint`.`Stu`, CONSTRAINT `Stu_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `Class` (`id`))