mysql not null default / default-优快云博客

本文介绍了MySQL中字段定义时使用default设置默认值的方法及not null约束的应用。通过实例演示了如何创建表并插入数据，展示了不同情况下字段值的表现。

not null default 说明不能是NULL，并设置默认值

default 设置默认值，但值也可能是NULL

mysql> create table test (id int, name varchar(10) default 'a', addr varchar(10)
 not null default 'b');
Query OK, 0 rows affected (0.04 sec)

mysql> desc test;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(10) | YES  |     | a       |       |
| addr  | varchar(10) | NO   |     | b       |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> insert into test id values(1);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MariaDB server version for the right syntax to use near 'id
values(1)' at line 1
mysql> insert into test id values(1);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MariaDB server version for the right syntax to use near 'id
values(1)' at line 1
mysql> insert into test(id) values (1);
Query OK, 1 row affected (0.00 sec)

mysql> insert into test(id) values (1);
Query OK, 1 row affected (0.00 sec)

mysql> insert into test(id) values (1);
Query OK, 1 row affected (0.00 sec)

mysql> select * from test;
+------+------+------+
| id   | name | addr |
+------+------+------+
|    1 | a    | b    |
|    1 | a    | b    |
|    1 | a    | b    |
+------+------+------+
3 rows in set (0.00 sec)

mysql> insert into test(id,name) values (1,null);
Query OK, 1 row affected (0.00 sec)

mysql> select * from test;
+------+------+------+
| id   | name | addr |
+------+------+------+
|    1 | a    | b    |
|    1 | a    | b    |
|    1 | a    | b    |
|    1 | NULL | b    |
+------+------+------+
4 rows in set (0.00 sec)

mysql> insert into test(id,addr) values (1,null);
ERROR 1048 (23000): Column 'addr' cannot be null
mysql>

转载于:https://www.cnblogs.com/perl6/p/7057616.html