我记得Oracle里不能为null
但在MySql里外键约束的字段为什么可以为null啊
groupid references B (id)啊~~
A
+---------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(255) | YES | | NULL | |
| groupid | int(11) | YES | MUL | NULL | |
+---------+--------------+------+-----+---------+----------------+B
+----+------+---------+
| id | name | groupid |
+----+------+---------+
| 1 | 男人 | NULL |
| 3 | 男人 | 2 |
+----+------+---------+
但在MySql里外键约束的字段为什么可以为null啊
groupid references B (id)啊~~
A
+---------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(255) | YES | | NULL | |
| groupid | int(11) | YES | MUL | NULL | |
+---------+--------------+------+-----+---------+----------------+B
+----+------+---------+
| id | name | groupid |
+----+------+---------+
| 1 | 男人 | NULL |
| 3 | 男人 | 2 |
+----+------+---------+
Mysql里边的也可以为空。