Self referencing foreign key - mysql not set to null - mysql

Question

Indexes :
Keyname Type Unique Packed Column Cardinality Collation Null
parent_id BTREE No No parent_id 1 A YES
Table : (comments)
Column Type Null Default Extra
id int(11) No None AUTO_INCREMENT
parent_id int(11) Yes NULL
Relation view:
Column Foreign key constraint (INNODB)
parent_id 'test_site'.'comments'.'id' ON DELETE CASCADE ON UPDATE NO ACTION
Is it possible to have parent_id not set to NULL. I have tried setting the default value to '0' and inserting the value '0' but I get the following error.
Error:
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)
Any help on this would be much appreciated, Thank you.

Answer 1

Yes, it is possible, although you have to circumvent the foreign key constraint just once to insert a dummy record for the default value. Here's my workflow:
Here's the table creation:
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)
Now circumvent foreign key and insert dummy record.
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)
To make things neat and tidy I reset auto_increment (this is not necessary):
root#localhost:playground > alter table comments auto_increment=0;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
And from now on your foreign key constraint is working properly and your column is no longer nullable and has a default value:
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)