Foreign key and the primary key in the same table - mysql

Question

I need to create a relationship between two attributes in the same table. So both primary key and the foreign keys are in the same table. Here I have a table called User_Type. Primary key is User_ID. It should be the foreign key of Parent_ID.
Ex:
User_Type
User_ID
User_Name
Parent_ID
User_Type_Division
But when I'm creating the relationship I get an error like this.
Cannot add or update a child row: a foreign key constraint fails
(mydb.user_type, CONSTRAINT Parent_User_Type FOREIGN KEY
(Parent_ID) REFERENCES user_type (User_ID) ON DELETE NO ACTION
ON UPDATE NO ACTION)").
Is there any way available to avoid this error. Please someone let me know.
And Here I have given the Query of the table.
CREATE TABLE IF NOT EXISTS `user_type` (
`User_ID` int(11) NOT NULL AUTO_INCREMENT,
`User_Name` varchar(45) NOT NULL,
`Parent_ID` int(11) DEFAULT NULL,
`User_Type_Division` varchar(45) DEFAULT NULL,
`User_ID_Format` varchar(45) DEFAULT NULL,
`Data_Entered_Person` varchar(45) DEFAULT NULL,
`Entered_Time` varchar(45) DEFAULT NULL,
PRIMARY KEY (`User_ID`),
UNIQUE KEY `User_Name_UNIQUE` (`User_Name`),
KEY `ParentUserType` (`Parent_ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=2 ;

Answer 1

There is a way to add and enforce foreign key on existing data.
Step 1: You need to stop foreign key checks for the current session.
SET FOREIGN_KEY_CHECKS=0;
Step 2: Add foreign key to your table.
ALTER TABLE myTable
ADD CONSTRAINT fk_name FOREIGN KEY ( columnName ) REFERENCES ...
Step 3: Enable foreign key checks.
SET FOREIGN_KEY_CHECKS=1;
Working Example:
mysql> create table fkchk( i int not null primary key auto_increment, n int );
Query OK, 0 rows affected (0.26 sec)
mysql> insert into fkchk(n) values ( 0 );
Query OK, 1 row affected (0.10 sec)
mysql> show variables like '%fore%';
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| foreign_key_checks | ON |
+--------------------+-------+
1 row in set (0.00 sec)
mysql> alter table fkchk
> add constraint fk_n foreign key (n) references fkchk(i)
> on delete no action;
ERROR 1452 (23000): Cannot add or update a child row:
a foreign key constraint fails
(`test`.<result 2 when explaining filename '#sql-6fc_14'>,
CONSTRAINT `fk_n` FOREIGN KEY (`n`)
REFERENCES `fkchk` (`i`) ON DELETE NO ACTION)
mysql> SET FOREIGN_KEY_CHECKS=0;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like '%fore%';
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| foreign_key_checks | OFF |
+--------------------+-------+
1 row in set (0.00 sec)
mysql> alter table fkchk
> add constraint fk_n foreign key (n) references fkchk(i)
> on delete no action;
Query OK, 0 rows affected (0.50 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> insert into fkchk(n) values ( 0 );
Query OK, 1 row affected (0.32 sec)
mysql> select * from fkchk;
+---+------+
| i | n |
+---+------+
| 1 | 0 |
| 2 | 0 |
+---+------+
2 rows in set (0.00 sec)
mysql> SET FOREIGN_KEY_CHECKS=1;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like '%fore%';
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| foreign_key_checks | ON |
+--------------------+-------+
1 row in set (0.00 sec)
mysql> insert into fkchk(n) values ( 1 );
Query OK, 1 row affected (0.12 sec)
mysql> insert into fkchk(n) values ( 0 );
ERROR 1452 (23000): Cannot add or update a child row:
a foreign key constraint fails
(`test`.`fkchk`,
CONSTRAINT `fk_n` FOREIGN KEY (`n`)
REFERENCES `fkchk` (`i`) ON DELETE NO ACTION)
mysql>
mysql> select * from fkchk;
+---+------+
| i | n |
+---+------+
| 1 | 0 |
| 2 | 0 |
| 3 | 1 |
+---+------+
3 rows in set (0.00 sec)
mysql>

Answer 2

Double check that the User_ID column and the Parent_ID column are of the same datatype.
Also, your User_ID key column is set to NOT NULL, but your Parent_ID column is set to null. that could be the problem. Set Parent_ID to NOT NULL and then try creating the FK relationship again. This is one of the options I was talking about. However, if you already have data in your table, this could be an issue too.