一 约束条件介绍：
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
二 not null与default
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(
nid int not null defalut 2,
num int not null
)
1 null
mysql> create table t1(id int);
Query OK, 0 rows affected (0.50 sec)
mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
1 row in set (0.01 sec)
mysql> insert into t1 values();
Query OK, 1 row affected (0.10 sec)
2 not null
mysql> create table t2(id int not null);
Query OK, 0 rows affected (0.64 sec)
mysql> desc t2;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)
再次插入空报错：
mysql> insert into t2 values();
ERROR 1364 (HY000): Field 'id' doesn't have a default value
3 defalut
#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
mysql> create table t3(id int default 1);
mysql> insert into t3 values();
Query OK, 1 row affected (0.09 sec)
mysql> select * from t3;
+----+
| id |
+----+
| 1 |
+----+
1 row in set (0.00 sec)
可以再次更改：
mysql> alter table t3 modify id int not null default 1;
三 unique唯一，不重复
1 uniqe:
创建一个表
mysql> create table department1(
-> id int,
-> name varchar(20) unique,
-> comment varchar(100)
-> );
Query OK, 0 rows affected (0.69 sec)
插入数据
mysql> insert into department1 values(1,'IT','技术');
Query OK, 1 row affected (0.10 sec)
再次插入数据报错：
mysql> insert into department1 values(1,'IT','技术');
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'
2 uniqe和not null 相当于主键 primary key
mysql> create table t4(id int not null unique);
Query OK, 0 rows affected (0.61 sec)
mysql> desc t4;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
+-------+---------+------+-----+---------+-------+
1 row in set (0.01 sec)
3 联合唯一 unique
mysql> create table service(
-> id int primary key auto_increment,
-> name varchar(20),
-> host varchar(15) not null,
-> port int not null,
-> unique(host,port) #联合唯一
-> );
Query OK, 0 rows affected (0.69 sec)
插入数据：
insert into service values(1,'nginx','192.168.0.10',80),(2,'haproxy','192.168.0.20',80),(3,'mysql','192.168.0.30',3306);
再次插入报错：
mysql> insert into service(name,host,port) values('nginx','192.168.0.10',80);
ERROR 1062 (23000): Duplicate entry '192.168.0.10-80' for key 'host'
四 primary key
从约束角度看primary key字段的值不为空且唯一，那我们直接使用not null+unique不就可以了吗，要它干什么？
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，一张表中必须有且只有一个主键。
一个表中可以：
单列做主键
多列做主键（复合主键）
1 单列做主键
方法1：
mysql> create table t5(id int not null unique,name varchar(20) not null unique,comment varchar(100));
Query OK, 0 rows affected (0.63 sec)
mysql> desc t5;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | NO | UNI | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
方法2：在某一个字段后用primary key
mysql> create table t6(id int primary key unique,name varchar(20),comment varchar(100));
Query OK, 0 rows affected (0.70 sec)
mysql> desc t6;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
方法3：在所有字段后单独定义primary key
mysql> create table t7(id int,name varchar(20),comment varchar(100),constraint pk_name primary key(id));#创建主键并为其命名pk_name
Query OK, 0 rows affected (0.64 sec)
mysql> desc t7;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
| comment | varchar(100) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
2 多列做主键（复合主键）
mysql> create table t8(ip varchar(15),port char(5),service_name varchar(10) not null,primary key(ip,port));
Query OK, 0 rows affected (0.58 sec)
mysql> desc t8;
+--------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| ip | varchar(15) | NO | PRI | NULL | |
| port | char(5) | NO | PRI | NULL | |
| service_name | varchar(10) | NO | | NULL | |
+--------------+-------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
插入数据：
mysql> insert into t8 values('172.16.45.10','3306','mysqld'),('172.16.45.11','3306','mariadb');
Query OK, 2 rows affected (0.33 sec)
Records: 2 Duplicates: 0 Warnings: 0
再次插入数据报错：
mysql> insert into service values ('172.16.45.10','3306','nginx');
ERROR 1136 (21S01): Column count doesn't match value count at row 1
五 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
1 不指定id
mysql> create table student(id int primary key auto_increment,name varchar(20),sex enum('male','female') default 'male');
Query OK, 0 rows affected (0.96 sec)
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 | |
+-------+-----------------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)
mysql> insert into student(name) values
-> ('egon'),
-> ('alex')
-> ;
Query OK, 2 rows affected (0.35 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 1 | egon | male |
| 2 | alex | male |
+----+------+------+
2 rows in set (0.00 sec)
2 指定id
mysql> insert into student values(4,'taibai','female');
Query OK, 1 row affected (0.33 sec)
mysql> select * from student;
+----+--------+--------+
| id | name | sex |
+----+--------+--------+
| 1 | egon | male |
| 2 | alex | male |
| 4 | taibai | female |
+----+--------+--------+
3 rows in set (0.00 sec)
3 删除id后，会不会从新开始自增？
#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, 3 rows affected (0.37 sec)
mysql> select * from student;
Empty set (0.00 sec)
mysql> insert into student(name) values('ysb');
Query OK, 1 row affected (0.09 sec)
mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 5 | ysb | male |
+----+------+------+
1 row in set (0.00 sec)
#如果用truncate是接清空表，那么表的id会从新开始计算
例子：
mysql> truncate student;
Query OK, 0 rows affected (0.54 sec)
mysql> select * from student;
Empty set (0.00 sec)
mysql> insert into student(name) values('egon');
Query OK, 1 row affected (0.33 sec)
mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 1 | egon | male |
+----+------+------+
1 row in set (0.00 sec)
六 foreign key 用于关联
1-1 快速理解foreign key
1 创建父表
#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
mysql> create table department(
-> id int primary key,
-> name varchar(20) not null
-> )engine=innodb;
Query OK, 0 rows affected (0.60 sec)
2 创建子表：
#dpt_id外键，关联父表（department主键id），同步更新，同步删除
mysql> create table employee(
-> id int primary key,
-> name varchar(20) not null,
-> dpt_id int,
-> constraint fk_name foreign key(dpt_id)
-> references department(id)
-> on delete cascade
-> on update cascade
-> )engine=innodb;
Query OK, 0 rows affected (0.34 sec)
分别再插入数据：
mysql> insert into department values
-> (1,'欧德博爱技术有限事业部'),
-> (2,'艾利克斯人力资源部'),
-> (3,'销售部');
Query OK, 3 rows affected (0.09 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> insert into employee values
-> (1,'egon',1),
-> (2,'alex1',2),
-> (3,'alex2',2),
-> (4,'alex3',2),
-> (5,'李坦克',3),
-> (6,'刘飞机',3),
-> (7,'张火箭',3),
-> (8,'林子弹',3),
-> (9,'加特林',3)
-> ;
Query OK, 9 rows affected (0.19 sec)
Records: 9 Duplicates: 0 Warnings: 0
#删父表department，子表employee中对应的记录跟着删
mysql> delete from department where id=3;
Query OK, 1 row affected (0.45 sec)
mysql> select * from employee;
+----+-------+--------+
| id | name | dpt_id |
+----+-------+--------+
| 1 | egon | 1 |
| 2 | alex1 | 2 |
| 3 | alex2 | 2 |
| 4 | alex3 | 2 |
+----+-------+--------+
4 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=22222 where id=2;
Query OK, 1 row affected (0.37 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from employee;
+----+-------+--------+
| id | name | dpt_id |
+----+-------+--------+
| 1 | egon | 1 |
| 2 | alex1 | 22222 |
| 3 | alex2 | 22222 |
| 4 | alex3 | 22222 |
+----+-------+--------+
4 rows in set (0.00 sec)
2-1 如何找出两张表之间的关系
分析步骤：
#1、先站在左表的角度去找
是否左表的多条记录可以对应右表的一条记录，如果是，则证明左表的一个字段foreign key 右表一个字段（通常是id）
#2、再站在右表的角度去找
是否右表的多条记录可以对应左表的一条记录，如果是，则证明右表的一个字段foreign key 左表一个字段（通常是id）
#3、总结：
#多对一：
如果只有步骤1成立，则是左表多对一右表
如果只有步骤2成立，则是右表多对一左表
#多对多
如果步骤1和2同时成立，则证明这两张表时一个双向的多对一，即多对多,需要定义一个这两张表的关系表来专门存放二者的关系
#一对一:
如果1和2都不成立，而是左表的一条记录唯一对应右表的一条记录，反之亦然。这种情况很简单，就是在左表foreign key右表的基础上，将左表的外键字段设置成unique即可
