一，介绍
约束条件与数据类型的宽度一样，都是任选参数。
作用：用于保证数据的一致和一致性。
主要划分为：
PRIMARY KEY（PK）标识该分区为该表的主键，可以唯一的标识记录
FOREIGN KEY（FK）标识该为该表的外键
NOT NULL标识该标识不能为空
UNIQUE KEY（UK）标识该细分的值是 唯一的
AUTO_INCREMENT标识该细分的值自动增长（整体类型，而且主要键）
DEFAULT为该分区设置值
UNSIGNED无符号
ZEROFILL使用0填充
二，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(
nid int not null defalut 2,
num int not null
)
练习：
1.创建可以插入空值的表1：
create table t1(id int);
desc t1;
insert into t1 values();
select * from t1;
mysql> create table t1(id int);
Query OK, 0 rows affected (0.84 sec)
mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
1 row in set (0.13 sec)
mysql> insert into t1 values();
Query OK, 1 row affected (0.22 sec)
mysql> select * from t1;
+------+
| id |
+------+
| NULL |
+------+
1 row in set (0.00 sec)
2.创建不能插入空值的表2：
create table t2(id int not null);
desc t2;
insert into t2 values();
执行结果为：
mysql> create table t2(id int not null);
Query OK, 0 rows affected (0.63 sec)
mysql> desc t2;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)
mysql> insert into t2 values();
ERROR 1364 (HY000): Field 'id' doesn't have a default value
3.创建有默认值的表3：
create table t3(id int not null default 1);
desc t3;
insert into t3 values();
insert into t3 values(2);
select * from t3;
执行结果为：
mysql> create table t3(id int not null default 1);
Query OK, 0 rows affected (0.49 sec)
mysql> desc t3;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | NO | | 1 | |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)
mysql> insert into t3 values();
Query OK, 1 row affected (0.47 sec)
mysql> insert into t3 values(2);
Query OK, 1 row affected (0.16 sec)
mysql> select * from t3;
+----+
| id |
+----+
| 1 |
| 2 |
+----+
2 rows in set (0.00 sec)
三，unique
设置唯一约束条件，设置后不可重复。
练习：
1.创建id唯一，学号唯一的表4：
create table t4(
id int unique,
name varchar(20),
number char(10),
unique(number));
insert into t4 values(1, "Alex", "18023300");
insert into t4 values(2, "Coco", "18023300");
insert into t4 values(2, "Coco", "19205200");
select * from t4;
执行结果为：
mysql> create table t4(
-> id int unique,
-> name varchar(20),
-> number char(10),
-> unique(number));
Query OK, 0 rows affected (0.93 sec)
mysql> insert into t4 values(1, "Alex", "18023300");
Query OK, 1 row affected (0.20 sec)
mysql> insert into t4 values(1, "Coco", "18023300");
ERROR 1062 (23000): Duplicate entry '1' for key 'id'
mysql> insert into t4 values(2, "Coco", "18023300");
ERROR 1062 (23000): Duplicate entry '1' for key 'number '
mysql> insert into t4 values(2, "Coco", "19205200");
Query OK, 1 row affected (0.09 sec)
mysql> select * from t4;
+------+------+----------+
| id | name | number |
+------+------+----------+
| 1 | Alex | 18023300 |
| 2 | Coco | 19205200 |
+------+------+----------+
2 rows in set (0.01 sec)
如果某些字段不为空且唯一，也就是不是null + unique，会定义为主键：
mysql> create table t5(id int not null unique);
Query OK, 0 rows affected (0.54 sec)
mysql> desc t5;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | 整数（11 ） | 否 | PRI | NULL |
+-------+---------+------+-----+---------+-------+