Self-Referential Table Fields In MySQL

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Self-Referential Table Fields In MySQL – This article will take you through the common SQL errors that you might encounter while working with sql, mysql,  phpmyadmin. The wrong arrangement of keywords will certainly cause an error, but wrongly arranged commands may also be an issue. SQL keyword errors occur when one of the words that the SQL query language reserves for its commands and clauses is misspelled. If the user wants to resolve all these reported errors, without finding the original one, what started as a simple typo, becomes a much bigger problem.

SQL Problem :

I have a table which has a “foreign key” referencing itself. This would be very useful, except I am uncertain how to add the first record to such a table. No matter what I add, I cannot provide a valid “foreign” key to the table itself, having no entries yet. Maybe I’m not going about this correctly, but I want this table to represent something that is always a member of itself. Is there a way to “bootstrap” such a table, or another way to go about self-reference?

Solution :

One option is to make your field NULL-able, and set the root record’s parent key to NULL:

CREATE TABLE tb_1 (
   id       int   NOT NULL  PRIMARY KEY,
   value    int   NOT NULL,
   parent   int   NULL,
   FOREIGN KEY (parent) REFERENCES tb_1(id)
) ENGINE=INNODB;
Query OK, 0 rows affected (0.43 sec)

-- This fails:
INSERT INTO tb_1 VALUES (1, 1, 0);
ERROR 1452 (23000): A foreign key constraint fails.

-- This succeeds:
INSERT INTO tb_1 VALUES (1, 1, NULL);
Query OK, 1 row affected (0.08 sec)

Otherwise you could still use a NOT NULL parent key and point it to the root record itself:

CREATE TABLE tb_2 (
   id       int   NOT NULL  PRIMARY KEY,
   value    int   NOT NULL,
   parent   int   NOT NULL,
   FOREIGN KEY (parent) REFERENCES tb_2(id)
) ENGINE=INNODB;
Query OK, 0 rows affected (0.43 sec)

-- This fails:
INSERT INTO tb_2 VALUES (1, 1, 0);
ERROR 1452 (23000): A foreign key constraint fails.

-- This succeeds:
INSERT INTO tb_2 VALUES (1, 1, 1);
Query OK, 1 row affected (0.08 sec)

You could do:

SET FOREIGN_KEY_CHECKS = 0;

Then perform the insert and then, set it back to 1 after. It is a session variable though so a disconnect will reset it, and it will not affect other connections.

Finding SQL syntax errors can be complicated, but there are some tips on how to make it a bit easier. Using the aforementioned Error List helps in a great way. It allows the user to check for errors while still writing the project, and avoid later searching through thousands lines of code.