自引用外键-mysql未设置为null

索引：

Keyname    Type   Unique  Packed  Column     Cardinality Collation  Null        
parent_id  BTREE  No      No      parent_id  1           A          YES 

表：（评论）

关系视图：

Column     Foreign key constraint (INNODB)
parent_id  'test_site'.'comments'.'id'  ON DELETE CASCADE   ON UPDATE  NO ACTION

是否可以将父\u id设置为空。我已尝试将默认值设置为“0”并插入值“0”，但出现以下错误

错误：

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update 
a child row: a foreign key constraint fails (`test_site`.`comments`, 
CONSTRAINT `comments_ibfk_2` FOREIGN KEY (`parent_id`) REFERENCES `comments` 
(`id`) ON DELETE CASCADE ON UPDATE NO ACTION)

---

非常感谢您在这方面提供的任何帮助。

是的，这是可能的，尽管您只需绕过外键约束一次即可插入默认值的虚拟记录。以下是我的工作流程：

以下是表的创建：

root@localhost:playground > create table comments(id int auto_increment primary key, parent_id int not null default 0, constraint fk_parent_id foreign key (parent_id) references comments(id) on delete cascade on update cascade)engine=innodb;
Query OK, 0 rows affected (0.01 sec)

root@localhost:playground > show create table comments\G
*************************** 1. row ***************************
       Table: comments
Create Table: CREATE TABLE `comments` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `parent_id` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `fk_parent_id` (`parent_id`),
  CONSTRAINT `fk_parent_id` FOREIGN KEY (`parent_id`) REFERENCES `comments` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

现在绕过外键并插入虚拟记录

root@localhost:playground > set session foreign_key_checks=0;
Query OK, 0 rows affected (0.00 sec)

root@localhost:playground > insert into comments (id) values (null);                                                                                              Query OK, 1 row affected (0.00 sec)

root@localhost:playground > set session foreign_key_checks=1;
Query OK, 0 rows affected (0.00 sec)

root@localhost:playground > select * from comments;
+----+-----------+
| id | parent_id |
+----+-----------+
|  1 |         0 |
+----+-----------+
1 row in set (0.00 sec)

root@localhost:playground > update comments set id = 0 where id = 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

root@localhost:playground > select * from comments;
+----+-----------+
| id | parent_id |
+----+-----------+
|  0 |         0 |
+----+-----------+
1 row in set (0.00 sec)

为了使事物整洁，我重置了自动增量（这不是必需的）：

从现在起，外键约束工作正常，列不再可为null，并具有默认值：

root@localhost:playground > insert into comments (id) values (null);
Query OK, 1 row affected (0.00 sec)

root@localhost:playground > select * from comments;
+----+-----------+
| id | parent_id |
+----+-----------+
|  0 |         0 |
|  1 |         0 |
+----+-----------+
2 rows in set (0.00 sec)

---

这是因为它需要引用一个真正的ID，或者只是null。因为您已经通过数据库的外观在数据库中建立了关系。如果我插入了一个默认行并将其用作引用ID，这会起作用吗？可能会，但为什么不摆脱数据库上的约束并确保应用程序覆盖逻辑呢？可以，我只是想知道这是否可能，我将让应用程序完成这项工作。谢谢。@David可能是因为忽略或忽略应用层的实现约束太容易了，并且有效地将约束从数据模型中移除。

root@localhost:playground > alter table comments auto_increment=0;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

root@localhost:playground > insert into comments (id) values (null);
Query OK, 1 row affected (0.00 sec)

root@localhost:playground > select * from comments;
+----+-----------+
| id | parent_id |
+----+-----------+
|  0 |         0 |
|  1 |         0 |
+----+-----------+
2 rows in set (0.00 sec)