表的完整性约束

表的完整性约束

约束条件和宽度一样，都是可选参数。用于保证数据的完整性和一致性。

分类

1PRIMARY KEY (PK)    标识该字段为该表的主键，可以唯一的标识记录
2FOREIGN KEY (FK)    标识该字段为该表的外键
3NOT NULL    标识该字段不能为空
4UNIQUE KEY (UK)    标识该字段的值是唯一的
5AUTO_INCREMENT    标识该字段的值自动增长（整数类型，而且为主键）
6DEFAULT    为该字段设置默认值
7UNSIGNED 无符号
8ZEROFILL 使用0填充

详细说明

11. 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
22. 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
3    sex enum('male','female') not null default 'male'
4     age int unsigned NOT NULL default 20 必须为正值（无符号） 不允许为空 默认是20
53. 是否是key
6   主键 primary key
7   外键 foreign key
8   索引 (index,unique...)

not null 与 default

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

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
create table tb1(id int not null defalut 2,num int not null)

 1==================not null====================
 2mysql> create table t1(id int); #id字段默认可以插入空
 3mysql> desc t1;
 4+-------+---------+------+-----+---------+-------+
 5| Field | Type    | Null | Key | Default | Extra |
 6+-------+---------+------+-----+---------+-------+
 7| id    | int(11) | YES  |     | NULL    |       |
 8+-------+---------+------+-----+---------+-------+
 9mysql> insert into t1 values(); #可以插入空
10mysql> create table t2(id int not null); #设置字段id不为空
11mysql> desc t2;
12+-------+---------+------+-----+---------+-------+
13| Field | Type    | Null | Key | Default | Extra |
14+-------+---------+------+-----+---------+-------+
15| id    | int(11) | NO   |     | NULL    |       |
16+-------+---------+------+-----+---------+-------+
17mysql> insert into t2 values(); #不能插入空
18ERROR 1364 (HY000): Field 'id' doesn't have a default value
19

1==================default====================
2#设置id字段有默认值后，则无论id字段是null还是not null，都可以插入空，插入空默认填入default指定的默认值
3mysql> create table t3(id int default 1);
4mysql> alter table t3 modify id int not null default 1;

 1==================综合练习====================
 2mysql> create table student(
 3    -> name varchar(20) not null,
 4    -> age int(3) unsigned not null default 18,
 5    -> sex enum('male','female') default 'male',
 6    -> hobby set('play','study','read','music') default 'play,music'
 7    -> );
 8mysql> desc student;
 9+-------+------------------------------------+------+-----+------------+-------+
10| Field | Type                               | Null | Key | Default    | Extra |
11+-------+------------------------------------+------+-----+------------+-------+
12| name  | varchar(20)                        | NO   |     | NULL       |       |
13| age   | int(3) unsigned                    | NO   |     | 18         |       |
14| sex   | enum('male','female')              | YES  |     | male       |       |
15| hobby | set('play','study','read','music') | YES  |     | play,music |       |
16+-------+------------------------------------+------+-----+------------+-------+
17mysql> insert into student(name) values('egon');
18mysql> select * from student;
19+------+-----+------+------------+
20| name | age | sex  | hobby      |
21+------+-----+------+------------+
22| egon |  18 | male | play,music |
23+------+-----+------+------------+
24

unique

限制字段的值唯一

1#单列唯一
2create table t16(
3    id int unique,
4    name char(16)
5);

1# 联合唯一
2create table server(
3    id int unique,
4    ip char(15),
5    port int,
6    unique(ip,port)
7);

primary key

主键primary key是innodb存储引擎组织数据的依据，innodb称之为索引组织表
单单从约束角度去看，primary key就等同于not null unique

强调
1、一张表中必须有，并且只能有一个主键
2、一张表中都应该有一个id字段，而且应该把id字段做成主键

1#单列主键
2create table t17(
3    id int primary key,
4    name char(16),
5    age int,
6    sex char(6)
7)engine=innodb;

1#多列主键(联合主键)
2create table t19(
3    ip char(15),
4    port int,
5    primary key(ip,port)
6);

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

auto_increment注意点：
1、通常与primary key连用，而且通常是给id字段加
2、auto_increment只能给被定义成key（unique key，primary key）的字段加

 1create table student(
 2id int primary key auto_increment,
 3name varchar(20),
 4sex enum('male','female') default 'male'
 5);
 6mysql> desc student;
 7+-------+-----------------------+------+-----+---------+----------------+
 8| Field | Type                  | Null | Key | Default | Extra          |
 9+-------+-----------------------+------+-----+---------+----------------+
10| id    | int(11)               | NO   | PRI | NULL    | auto_increment |
11| name  | varchar(20)           | YES  |     | NULL    |                |
12| sex   | enum('male','female') | YES  |     | male    |                |
13+-------+-----------------------+------+-----+---------+----------------+
14mysql> insert into student(name) values
15    -> ('egon'),
16    -> ('alex')
17    -> ;
18mysql> select * from student;
19+----+------+------+
20| id | name | sex  |
21+----+------+------+
22|  1 | egon | male |
23|  2 | alex | male |
24+----+------+------+

 1#也可以指定id
 2mysql> insert into student values(4,'asb','female');
 3Query OK, 1 row affected (0.00 sec)
 4mysql> insert into student values(7,'wsb','female');
 5Query OK, 1 row affected (0.00 sec)
 6mysql> select * from student;
 7+----+------+--------+
 8| id | name | sex    |
 9+----+------+--------+
10|  1 | egon | male   |
11|  2 | alex | male   |
12|  4 | asb  | female |
13|  7 | wsb  | female |
14+----+------+--------+

 1#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
 2mysql> delete from student;
 3Query OK, 4 rows affected (0.00 sec)
 4mysql> select * from student;
 5Empty set (0.00 sec)
 6mysql> insert into student(name) values('ysb');
 7mysql> select * from student;
 8+----+------+------+
 9| id | name | sex  |
10+----+------+------+
11|  8 | ysb  | male |
12+----+------+------+

 1#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
 2mysql> truncate student;
 3Query OK, 0 rows affected (0.01 sec)
 4mysql> insert into student(name) values('egon');
 5Query OK, 1 row affected (0.01 sec)
 6mysql> select * from student;
 7+----+------+------+
 8| id | name | sex  |
 9+----+------+------+
10|  1 | egon | male |
11+----+------+------+
121 row in set (0.00 sec)
13