是否可空，null表示空，非字符串
not null - 不可空
null - 可空
use db4；
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
mysql> create table tb2(id int not null); Query OK, 0 rows affected (0.01 sec) mysql> desc tb2; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.00 sec) mysql> insert into tb2 values(); Query OK, 1 row affected, 1 warning (0.00 sec) mysql> select * from tb2; +----+ | id | +----+ | 0 | +----+ 1 row in set (0.00 sec)
==================not null====================
mysql> create table t1(id int); #id字段默认可以插入空 mysql> desc t1; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | +-------+---------+------+-----+---------+-------+ mysql> insert into t1 values(); #可以插入空
mysql> create table t2(id int not null); #设置字段id不为空 mysql> desc t2; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | +-------+---------+------+-----+---------+-------+ mysql> insert into t2 values(); #不能插入空 ERROR 1364 (HY000): Field ‘id‘ doesn‘t have a default value
==================default====================
#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 tb3(id int default 1); Query OK, 0 rows affected (0.01 sec) mysql> insert into tb3 values(); Query OK, 1 row affected (0.00 sec) mysql> select * from tb3; +------+ | id | +------+ | 1 | +------+ 1 row in set (0.00 sec)
原文：https://www.cnblogs.com/mingerlcm/p/9807321.html